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Dictionary of the History of Ideas

Studies of Selected Pivotal Ideas
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The problem of the certainty of knowledge was not
new in the seventeenth century. In the preceding cen-
tury the Reformation brought to the fore the question
of the reliability of religious knowledge, particularly
the question of which church—Catholic or Protestant—
could provide the best guarantee of the truth of its
doctrines. The revival of learning in the Renaissance
brought to the attention of the learned of the time
the writings of the ancient Greek skeptics, particularly
of Sextus Empiricus (ca. A.D. 200). Most deeply affected
by this discovery was Michel de Montaigne who in
his Apology for Raymond Sebond extended the argu-
ments of the skeptics to raise questions about the relia-
bility of all our knowledge. The issue involved in the
resurgence of skepticism was the question of how cer-
tain one can be of what he believes. The skeptic claims
one can have no knowledge at all; at best one must
remain in doubt and suspend judgment. If the structure
of nature is complex, if the senses sometimes deceive,
if one's thought processes do not match the structure
of nature, then nothing is certain. Montaigne asks: If
the senses cannot be trusted how reliable is scientific
knowledge? If reason errs how reliable are claims to
know of God's existence and nature? If moral principles
cannot be justified then how seriously ought one to
pursue the good life or try to do what is right?

If the problem of certainty was not new for the
seventeenth century, the solutions to it were. In addi-
tion to the suspension of judgment counselled by such
skeptics as François de La Mothe Le Vayer, Pierre
Charron, and François Veron, two alternative views
were developed. First there was the extreme opposite
view, that of dogmatism. Francis Bacon argued that
by furnishing the senses with mechanical aids and the
mind with a new method of inquiry one can gain
certain knowledge about nature. René Descartes ar-
gued that by grounding all knowledge on clear and
distinct ideas of the self and of God, completely certain
knowledge can be gained about self, God, and the
world. The second major alternative to skepticism was
the claim that a middle way could be found between
the extremes of skeptical uncertainty and complete
certainty, thereby forming a perspective which later
became the foundation of what we now call the “sci-
entific” temperament. This view has been called “con-
structive skepticism.” On the Continent this view was
initially worked out in the context of science by Marin
Mersenne and Pierre Gassendi. In England it was for-
mulated in the context of theology by a liberal Angli-
can, a convert from Catholicism, William Chilling-
worth, and later systematized and applied to the theory
of science by such early members of the Royal Society
of London as John Wilkins and Joseph Glanvill. It was
applied in the sciences by such men as Robert Boyle
and Sir Isaac Newton. Finally, it received its definitive
expression in the philosophical writings of John Locke.
However, it was not completely assimilated into the
mainstream of European thought until the dogmatism
of Descartes was dealt its final blow by David Hume.

Francis Bacon, the reputed father of modern experi-
mental science, was one of the first to formulate a way
out of the skeptical crisis. Though his emphasis upon
experimental procedures and on the utility of science
for the benefit of human life won him acclaim in his
own century, it was not his views on the certainty of
the results of scientific inquiry that were the decisive
ones in the shaping of the views of early members of
the Royal Society and of the scientific temperament.

The best known and perhaps most serious attempt
to solve the skeptical crisis of the seventeenth century


was formulated by René Descartes. He was aware that
the Reformation, the new developments in science, and
the resurgence of skepticism had thrown all accepted
claims to knowledge into question. He was personally
acquainted with such skeptics as Mersenne and Veron
and knew the arguments of Montaigne and Charron.
The view he found among his immediate acquaintances
was that everything was subject to doubt. Probability
was the most that could be claimed for anything.
Descartes realized that if there are only probabilities,
then there is no secure foundation for truth; for then
there is no adequate criterion by which to distinguish
truth from falsity. The strategy of his position is to
follow the arguments of the skeptics to their conclu-
sion, to reject everything as false, and in the depths
of uncertainty to find truth and a criterion of truth.
This criterion is then tied to the goodness of God.

In the Discourse on Method (1637) Descartes had
formulated a “natural method” to assure certainty in
the sciences. The first rule of this method was “...
to accept nothing as true which I did not clearly rec-
ognize to be so... to accept... nothing more than
what was presented to my mind so clearly and dis-
tinctly that I could have no occasion to doubt it.” Ten
years before publishing the Meditations (1641) he came
to realize that all knowledge was under attack and
therefore the criterion would have to be applied more
extensively. Indeed, it became a very definition of
knowledge: to know is to apprehend without any pos-
sibility of error. The Meditations begins with a re-
hearsal of basic skeptical arguments from the fallibility
of the senses and problems about being awake or
asleep. In rehearsing such arguments he is claiming
nothing particularly new; these were the standard
weapons of the skeptics. To accept them was to admit
that the senses were unreliable sources of knowledge
of the nature of the external world. Descartes realized
that to overcome skepticism he had to go all the way.
Hence he formulates an argument more critical than
any presented by any preceding skeptic: the argument
based on the possibility that there is an evil demon
whose sole business is to make us believe true what
may in fact be false. Unless this serious doubt is over-
come, skepticism will always triumph, for there is then
no dependable criterion of truth. Unless one can ex-
orcise the evil demon no criterion is possible; what
appears as self-evident may in fact be false. Even such
simple matters as that a square has four sides or that
three plus two is equal to five may be erroneous. Given
the possibility of the evil demon all evidence and the
rational processes of interpreting it are subject to error.
Hence what can be known? Descartes suggests that
his use of skeptical arguments differs from that of the
skeptics' use of them. Whereas they used them to show
that nothing can be known he is using them to show
precisely that truth and a criterion for its discovery
can be found. Descartes finds the answer to doubt in
the famous cogito ergo sum (“I think, therefore I am”).
In formulating the most critical of the arguments of
skepticism one discovers something true, his own exist-
ence. This follows not as the conclusion of an argument
but as the conclusion of a doubt.

Having found this one truth Descartes has also found
the criterion by which to distinguish truth from falsity:
whatever is as clearly and distinctly perceived as the
cogito is true. This criterion enables him by the light
of natural reason to recognize the truth of several
causal principles with which, in conjunction with the
idea of a perfect being, he can demonstrate the exist-
ence of a perfect being, God. This being serves two
functions: first, being perfect he would not allow man
always to be deceived, thus exorcising the evil demon;
and secondly, he guarantees the correctness of the
criterion. In short, the criterion is rooted in the good-
ness of God. From this point Descartes proceeds to
establish the reliability of the senses and prove the
existence of the physical world. On this metaphysics
of self, God, and matter, together with the criterion
of clarity and distinctness, he then rebuilds the scien-
tific description of the world around us. What we find
in Descartes, then, is the recognition that “The Refor-
mation, the revolution in science, and the onslaught
of scepticism had crumbled the old foundations that
used to support the entire framework of man's intel-
lectual achievements. A new age required a new foun-
dation to justify and guarantee what it had discovered.
Descartes, in the tradition of the great medieval minds,
sought to provide this basis by securing the superstruc-
ture, man's natural knowledge, to the strongest possible
foundation, the all-powerful, eternal God” (Popkin, p.

By Descartes' criterion—not to accept anything as
true unless it is clearly and distinctly perceived—not
only is one subjectively certain that something is true,
but he is certain that it actually is so. Without the
goodness of God there is always room for doubt. In
working out this solution to the skeptical problem
Descartes has offered, as Popkin argues, a Reformation
solution to an epistemological problem concerning our
secular knowledge. The Reformers and their opponents
had shown each other that their views were without
rational justification. The Reformers, particularly the
Calvinists, had then argued that by a direct insight they
were privileged to see true principles of faith and by
an act of divine grace to see that these principles agree
with the rule of faith, the Scriptures. What Descartes
offers is the same kind of argument for all, not merely
theological, knowledge. We perceive truth directly,


which in turn reveals a criterion; and by the criterion
one can discover the goodness of God which guarantees
the reliability of the criterion.

Descartes' solution to the skeptical crisis brought
him immediate acclaim, not only by his defenders but
also, and more particularly, by such critics as Mer-
senne, Gassendi, and Hobbes. But in spite of the sever-
ity of the attacks on his views, his solution to the
skeptical crisis pushed out of sight an alternative to
it, a solution which in the long run triumphed over
his own.

Constructive skepticism—the lasting contribution of
the seventeenth century to the “scientific attitude” and
to contemporary philosophy—admits the full force of
the skeptical attack on necessary and certain knowl-
edge, but allows for a lesser knowledge about appear-
ances. Mersenne and Gassendi on the Continent and
Chillingworth in England first formulated the view
later worked out by some early members of the Royal
Society of London and fully articulated by John Locke
but not accepted until the collapse of the dogmatism
of Descartes.

Marin Mersenne was trained at the Jesuit school at
La Flèche and later joined the Order of Minîmes. A
great part of his life was devoted to publicizing the
“new science.” His writings are an attempt to show
the importance of science despite the attacks of the
skeptics. His book, La Vérité des sciences (1625), argues
that even if the skeptic cannot be refuted we have a
wealth of knowledge suited to our purposes in life.
Such knowledge consists of information about appear-
ances: hypotheses can be made about the connections
of events and predictions made of the future course
of experience. Science and mathematics do not yield
knowledge about any transcendent reality or make any
metaphysical claims or depend upon any such claims,
as both Bacon and Descartes had maintained.

Mersenne is fully conscious of the skeptics' attack
on knowledge. His position is not that we do not know
anything but only that some things are unknown. Ap-
pearances and effects can be known, though we do not
know their real causes. Our senses do not inform us
about the real natures of things but this does not pre-
vent formulation of laws—such as those of refrac-
tion—which enable us to predict future experiences
and account for observed events. Despite the disagree-
ments men have, which Mersenne cannot but ac-
knowledge, there are many agreements among them
too, and it is this that the skeptic overlooks.

Against the claim of the skeptic that everything is
a matter of controversy Mersenne replies that many
things are never disputed. Against the claim that all
arguments lead to an infinite regress (in the search for
premisses to support premisses) Mersenne claims that
there are some self-evident premisses and that predic-
tions can be checked empirically to determine the
accuracy of the argument. In reply to the skeptic's
claim that syllogistic reasoning is unreliable because
there is no guarantee that any set of premisses guaran-
tees its conclusion, he answers that we do judge of the
truth of premisses and accept conclusions.

Mersenne's avowal of the extent of our knowledge
is not itself a claim that complete certainty can be
attained. Indeed, in his Les Questions théologiques
(1634) he presents his own skeptical views. First, he
shows that knowledge of eternal truths is not possible.
All we know is subject to doubt. There is no body of
demonstrative knowledge. He accepts the antimeta-
physical claims of the skeptic but also argues for the
truth of science. The reliability of knowledge does not
depend on discovering the grounds of all certainty. In
this sense both the dogmatist and the skeptic are
mistaken—the one for claiming that metaphysical
knowledge is possible and actual, the other for claiming
that it is not. Both are mistaken in assuming that sci-
ence depends upon a metaphysics and that metaphysi-
cal knowledge must be completely certain. Both the
dogmatist and the skeptic are undermined. Between
their positions lies a constructive path: doubting the
grounds but accepting the structure of knowledge.

Not unlike the views of Mersenne were those of
Pierre Gassendi. Known best as a critic of Descartes,
Gassendi actually rivalled him in popularity as a scien-
tist and philosopher of science. Well-read in the argu-
ments of the skeptics, Gassendi rejected them as de-
featist and negative because of the advances made in
the sciences in his own times. He is credited with
introducing the distinction between primary and sec-
ondary qualities of objects into modern philosophy,
thereby attacking the Aristotelian view that scientific
knowledge can both be based on sense perception and
be necessarily true. If the observable properties of
things are not their real properties, there can be no
necessarily true empirical knowledge. All we know are
the appearances of things. As a disciple of the skeptical
tradition Gassendi holds that all claims to knowledge
about the real world are unfounded and that appear-
ances are the basis for all knowledge. By experience
we cannot discover any general principles because we
may always discover a negative instance. We observe
only the appearances of things, not their real struc-
tures; hence we can make no inferences from appear-
ances to reality. However, to account for these phe-
nomena Gassendi reconstructed a version of Epicurean
atomism—not as a metaphysics but as a set of theoret-
ical constructs to account for appearances, a view
which Robert Boyle later characterized as the “cor-
puscularean hypothesis.” Such knowledge as we have


about appearances does not yield certainty about the
real structure of nature, though it gives a certainty
sufficient to make predictions about future experiences.
It is practically useful though not completely certain.

The views put forward by Mersenne and Gassendi
were generally not understood by their contem-
poraries; further, they were overshadowed by the
forcefulness of Descartes' position. It was not until a
view somewhat like theirs was developed in England
that their solution to the intellectual crisis of the time
could make its impact.

During the second and third decades of the century
there arose in England what has become known as “the
Rule of Faith controversy,” an outgrowth of the Refor-
mation controversy concerning the adequacy of each
church for salvation. Each side used a battery of argu-
ments from the arsenal of skepticism to show that the
other side could not guarantee its claim to religious
knowledge and truth. The Catholic polemicists argued
that their opponents had no way of determining which
book is the Bible, what it says, or what one ought to
do about it. Protestants argued that the appeal by
Catholics to tradition and authority was unreliable, and
that the average man had no way of determining in-
fallibly what the tradition was, the reliability of its
authority, nor even who was Pope.

Out of this controversy grew a form of constructive
or mitigated skepticism that was able to deal with the
issues. The view is offered that theoretically it might
not be possible to eradicate or overcome the doubts
raised about the possibility of religious knowledge, but
that there was a type of assurance that was sufficient
for practical religious purposes. By using the standards
of common sense and practical life one could attain
a limited amount of certainty—as much as the case
admits of—a certainty that is beyond any “reasonable
doubt” and sufficient for any “reasonable man.”

One of the leading contenders in the Rule of Faith
controversy was the young Anglican clergyman,
William Chillingworth, whose once famous book, The
Religion of Protestants, a Safe Way to Salvation
deals with the theological issue of whether Protestants
have any assurance of salvation outside the Catholic
Church. His opponents had argued that there are only
two alternatives with respect to religious knowledge:
either absolute certainty of the truths of faith or none
at all. With respect to the first, Chillingworth argues
that such certainty is not humanly attainable in reli-
gious matters, though possible in mathematics and
metaphysics. Skeptical arguments against the senses
and reason preclude such certainty. He is also un-
willing to accept the second alternative, skepticism;
for such arguments, if taken too seriously, can destroy
all belief. He proposes instead that one can be reasona
bly certain of the doctrines necessary for salvation. The
assurance of common sense about everyday affairs is
made the standard for solution to perplexities about
religion. The reasonable solution to any problem is to
examine such evidence as is available and to proportion
assent to it. It became a cardinal principle to propor-
tion assent to evidence: to each kind of evidence there
corresponds a kind or degree of assent, and as the
evidence is greater or less so should be one's certainty.

In his description of the kinds of certainty a person
can have of the truth of a statement Chillingworth is
not systematic. In the main he distinguishes three levels
with their subclassifications or degrees: (1) absolutely
infallible certainty, (2) conditionally infallible cer-
tainty, and (3) moral certainty. The first of these—
presumably the kind of certainty Descartes sought in
his Meditations—Chillingworth regards as beyond
human reach. Such certainty excludes every possibility
of doubt, a condition not attainable by mortal man.
Conditionally infallible certainty is the highest attain-
able by man; it is based upon such evidence as excludes,
for all human purposes, the possibility of error, but
does not exclude it completely since all experiences
may, for example, really be dream events. This kind
of certainty occurs whenever one has knowledge, that
is, in those instances in which, upon knowing the
meanings and connections of the terms involved, assent
is compelled rather than voluntary. Because such
mathematical truths as “the whole is greater than any
of its parts” and “twice two is four” are of this kind,
the certainty one has of their truth is called mathe-
matical certainty; similarly the axioms of metaphysics
are said to be metaphysically certain. Such infallible
certainty is possible not only for simple statements of
the kinds of mentioned, but for demonstrations also, pro-
vided the rules of deductive inference are correctly

The third kind of certainty is moral certainty, the
certainty one has of what is believed but not known.
This is the certainty of everyday life about matters of
fact and is based on such evidence as excludes the
possibility of error for all practical purposes. The cer-
tainty one has when traveling that he is on the right
road and that the book one is reading is the work of
the person whose name appears on the title page are
of this sort. Moral certainty is described as the certainty
a sane, reasonable, and thoughtful person has after
considering all available evidence as fully and impar-
tially as is possible and giving his assent to that side
on which the evidence seems strongest. Since belief
is based upon a different kind of evidence than knowl-
edge, and is therefore less certain, the possibility of
error is increased. As in conditionally infallible cer-
tainty the dream hypothesis prevents absolute certainty


from being attained, so in moral certainty a skeptical
doubt also operates, for “seeing the generality of men
is made up of particulars, and every particular man
may deceive and be deceived, it is not impossible,
though exceedingly improbable, that all men should
conspire to do so” (Chillingworth, pp. 203-04). If this
possibility were taken seriously life would soon fall
apart. If on grounds of possibility of error alone one
refused to accept the testimony of witnesses the trial
of criminal cases would soon become a shambles. Re-
jection on similar grounds of the reports of travelers
and chroniclers would soon turn history into fiction.
Although such possibilities exist, they are not destruc-
tive of the everyday activities of life; such beliefs as
one has of matters of fact—of routes, the reliability
of witnesses, and so on—suffice to get one through the
day. Many things of which one is only morally certain
may be true, but since the available evidence does not
warrant a claim to knowledge one need and can be
only as certain as the evidence allows. Unlike mathe-
matical certainty, moral certainty admits of degrees
because the strength of the evidence varies. Precisely
where the lines are to be drawn between the several
degrees of moral certainty, however, and what the
standard is by which the degrees are measured, are
not made clear.

The theoretical justification for accepting moral
certainty is that some things by their very natures do
not admit of more than a specific kind and amount
of evidence and therefore it is unreasonable to demand
more, e.g., mathematical proof for a matter of fact.
The practical justification, stated with respect to reli-
gious belief, is that moral certainty suffices to turn the
sinner from the ways of darkness to the path of light.

This view of the nature of certainty was developed
in the next generation by several early members of the
Royal Society of London who were also clergymen,
among them John Tillotson, Joseph Glanvill, and John
Wilkins. As so developed, especially by Wilkins and
Glanvill, the theory was secularized, i.e., stated and
applied in the context of scientific knowledge. Wilkins'
views are particularly instructive, for in them is found
a formulation in the context of religion which was
applied to scientific procedures by Royal Society
members and which later received full expression in
the writings of John Locke. Its application to scientific
procedures was due largely to Wilkins' influence in
founding the Royal Society and in formulating its

After taking a degree at Oxford in 1634 Wilkins took
orders and rose rapidly in the Church of England—in
part because of political influence. One of his major
interests was in science. Though not of the prominence
of such of his friends as Boyle, Hooke, Wallis, and
Barrow, he nevertheless took a keen interest in several
of the particular sciences. More important than his role
as an amateur scientist was his association with the
Royal Society. It is known that he attended scientific
meetings of the Invisible College in London as early
as 1645, and that he initiated a new series of meetings
in Oxford when he moved there. Later, in 1660, he
presided over the meeting in which formal action was
taken to establish a society. He held offices in and was
active in the Royal Society until his death.

Wilkins' major work, Of the Principles and Duties
of Natural Religion,
was published posthumously in
1675. In it he sees knowledge characterized by the
same extremes as had Chillingworth, absolute certainty
and extreme skepticism. He acknowledges the ideality
of the first but realizes that it cannot be attained be-
cause of the limitations of the human faculties. The
second alternative is at all costs to be avoided, because
acceptance of “the cavils of Sceptical captious Men”
would put an end to the certainty of religious and
scientific belief and of the beliefs of everyday life, none
of which Wilkins is willing to forego. He is not willing,
as was Descartes, to reject everything for which the
conditions of falsity can be stated; such skepticism leads
to nihilism and is therefore rejected as absurd. Wilkins
admits at the outset that the structure of reality cannot
be completely understood by the human mind, but does
not take this as a reason for despair.

The antiskeptical theme is an important one in his
book on religion, in which the major question for
consideration is, how can religious belief be justified
without capitulation to the arguments of the skeptic?
Wilkins' solution is a development of the theory of
probable certainty stated earlier by Chillingworth. In
general, he disregards the arguments of the skeptic and
bases his theory of religious knowledge on the fact that
ordinary people are not seriously affected by such
arguments. In their daily and practical affairs people
do claim certainty and base it on evidence. He tries
to discover what kinds of certainty they claim, together
with the corresponding kinds and levels of evidence.
In his classification, a scheme later used by Locke in
his Essay Concerning Human Understanding (IV, xiv-
xv), two levels of assent are distinguished. On the one
hand there is knowledge, “... which doth arise from
such plain and clear Evidence as doth not admit of
any reasonable cause of doubting” (Wilkins, p. 5).
Three subclassifications are made: physical, mathe-
matical, and moral certainty. On the other hand there
is probability, “... which doth arise from such Evi-
dence as is less plain and clear” (Wilkins, p. 8). Beneath
these, where the evidence is such as to warrant neither
affirmation nor denial, there is simply hesitation or
suspension of judgment.


On another scheme of classification, which follows
Chillingworth's text more closely, three levels of cer-
tainty are distinguished: absolutely infallible certainty,
conditionally infallible certainty, and indubitable cer-
tainty. The first is the prerogative of God. The second
comprises both physical and mathematical certainty
and is the greatest humanly possible (revelation possi-
bly excepted). Such certainty presupposes fulfillment
of two requirements, that “our faculties be true, and
that we do not neglect the exerting of them” (Wilkins,
p. 9). Wilkins sets down their fulfillment as a postulate
without which knowledge is impossible. The third
level, indubitable certainty, which is the same as moral
certainty, is not as strong as infallible certainty for,
although the same suppositions are made, the evidence
is less strong and thus provides only an assurance
“which doth not admit of any reasonable cause of

Each of these kinds of certainty is clarified by an
examination of the evidence to which each is supposed
to be correlated. In doing this Wilkins tries to be
precise and so defines his crucial terms, though his
meanings are not always clear. He begins with the
senses and distinguishes the external from the internal.
The senses are the source of the highest certainty of
which man is capable, physical certainty, but they are
not infallible. Wilkins admits that he does not know
how the senses operate, but claims that this is no
argument against the genuineness of the knowledge
derived from them. The causes of sense perception are
one thing, the results another, and difficulties about
the one do not necessarily bode ill for the other.

The second kind of evidence Wilkins considers—
evidence from the nature of things—relates to the
function of the understanding and (at its best) gives
rise to mathematical certainty. Mathematical certainty
is the kind of assurance one has of the propositions
and demonstrations of mathematics and other abstract
sciences. Upon knowing the meanings of the terms
involved no one could deny without contradiction that
the whole is greater than the part or that three plus
three equals six. There occurs here what Wilkins calls
“natural necessity”; one could no more deny this sort
of proposition than he could avoid feeling hungry or
sleepy. Denial of such propositions as these would be
self-contradictory for it would be to deny meanings
already accepted. Evidence from the nature of things
occurs “when there is such a Congruity or Incongruity
betwixt the Terms of a Proposition, or the Deduction
of one Proposition from another, as doth either satisfy
the mind, or else leaves it in doubt and hesitation about
them” (Wilkins, p. 4).

Moral or indubitable certainty follows mathematical
certainty. Of the three major kinds of certainty Wilkins
recognizes this is the most important since of most
things said to be known one can at best be only morally
certain. By it he runs the middle course between abso-
lute certainty and complete uncertainty. Moral cer-
tainty is the assurance one has of anything for which
there is no ground for a “reasonable doubt.” In making
reasonable doubt the distinguishing feature of moral
certainty Wilkins is making an appeal to common
sense. In the context of religion in which he is writing
the skeptic and dogmatist are both unreasonable, the
one for making the bare possibility of doubt a sufficient
reason for withholding assent, and the other for de-
manding a kind of evidence not open to any doubt
whatever. Wilkins makes an appeal to what the man
in the street believes, and makes this the standard by
which to solve his problems: whatever is unacceptable
to the common and reasonable man is probably not
true. This appeal is made in two ways. First, the exam-
ples he uses are drawn from the ordinary affairs of life:
the reports of travelers, merchants, and the like. “I
appeal to the common judgment of mankind,” says
Wilkins. Secondly, the kind of certainty appealed to
is that of the reasonable man. What an ordinary person,
possessed of all his faculties and judicious in his use
of evidence, would accept as true is for Wilkins the
standard of truth.

The principle of reasonable doubt leaves open the
possibility that what has been proved may be otherwise
than the evidence indicates; however, this is not a
sufficient reason for withholding assent. “Who is there
so wildly skeptical as to question,” asks Wilkins,
“whether the Sun shall rise in the East, and not in the
North, or West, or whether it shall rise at all?” No
one can doubt such things without abandoning ration-
ality. Acceptance of less than infallible certainty is a
principle so strong in human nature that if it were
otherwise reason would be a torment to mankind. Men
would be soon driven to insanity if the mere possibility
of error were a suitable basis on which to establish
one's doubts or beliefs.

The evidence giving rise to moral certainty is that
of the senses or of the nature of things when the senses
are not adequate to give rise to higher levels of cer-
tainty. Particularly relevant to belief in matters of fact
beyond one's immediate experience is testimony. The
testimony of witnesses to a crime, of an explorer to
the customs of a distant country, or of an historian
to events in the past are all adequate bases for belief,
provided that the witnesses are authoritative and cred-
ible. The criteria for determining competence are not,
however, spelled out by Wilkins.

In laying the groundwork for his discussion of natural
religion Wilkins proceeds, after considering the kinds
of evidence and certainty, to establish their relationship


more closely. One point he makes is that there are no
degrees of truth; truth does not admit of more or less.
Historical and geographical statements are as true as
those of mathematics and metaphysics. The problem
is that the same kind of evidence cannot be produced
for each. But to deny the truth of an historical proposi-
tion because it cannot be immediately verified in sense
experience or by demonstration (and thus not infallibly
certain) would involve the gross error of supposing that
because all truths are on a par the evidence must be
so too. Wilkins cites in this connection a passage from
Aristotle's Nicomachean Ethics (I, 3) in which it is
pointed out that where there are differences in the
subject matter to be proved the kind of evidence must
be expected to differ. Paraphrasing Aristotle, Wilkins
points out that it would be absurd to use a rhetorical
speech to prove a mathematical proposition and
equally out of place to use a demonstrative argument
to arouse the emotions of an audience. This principle
is Wilkins' ultimate justification for accepting less than
infallible certainty in religion and the sciences. So long
as any proof is good in its kind and suited to the subject
matter, its conclusion is justified.

The account of constructive skepticism worked out
by Chillingworth and Wilkins with respect to religious
knowledge was not restricted to that domain. There
is evidence to show that even in Wilkins' writings an
effort is made to extend it to scientific knowledge as
well. The writings of several of his contemporaries
contain views on evidence and certainty remarkably
similar to those Wilkins held. In the “experimental
philosophy” of the Royal Society scientific conclusions
are not taken to be absolutely true but as probable
only. Such conclusions are subject to revision upon
discovery of new evidence. The ideal of science as a
deductive system is recognized for what it is, an ideal
not attainable in practice because of the limitations
of our faculties and the complexity of nature. Science
does not make known the real structure of things but
formulates hypotheses to account for what is ob-
served—a redevelopment of the account put forward
by Mersenne and Gassendi.

This account of certainty as applied to scientific
conclusions was detailed by such early members of the
Royal Society as Joseph Glanvill and Robert Boyle.
Whereas Glanvill was primarily concerned with the
theory of science, Boyle applied this account of cer-
tainty to his own scientific conclusions. However, much
of Boyle's exposition of the theory is still in the context
of religion; the majority of his comments on the nature
and extent of knowledge occur in his theological writ-
ings. In the writings of Isaac Newton the situation is
somewhat different. Here there is little discussion of
a theory of knowledge though a theory is assumed in
his scientific writings. Whereas most of his contem-
poraries in the Royal Society had been explicit in their
views on the limitations of knowledge and the levels
of certainty, Newton has little to say on these points.
According to him there are some things the scientist
finds beyond the reach of intellect and whose natures
must be acknowledged as unknown, particularly the
natures of light and gravity and of physical bodies,
though speculative hypotheses can be formulated to
account for them.

In his early contributions to the Philosophical Trans-
concerning the analysis of light Newton says
that his views are certain; however, it is an account
only of the rays of light as they appear to the observer
and not as they are in themselves. “But, to determine
more absolutely, what light is, after what manner re-
fracted, and by what modes or actions it produceth
in our minds the Phantasms of Colours, is not so easie.
And I shall not mingle conjectures with certainties”
(Phil. Trans. No. 80 [19 February 1671/72]). To give
an account of the nature of light would be to formulate
a speculative hypothesis, a conjecture which has no
role in the presentation of the results of scientific in-
quiry. Though he sought to discover the nature of light
he was not successful in doing so. Hence he limited
himself to a descriptive account of how light is bent,
refracted, and so on.

As in the case of light, so also the force of gravity
is unknown. In spite of this Newton did manage to
formulate some significant laws describing the motions
of bodies as affected by gravity. That such significant
results can be achieved, it should be noted, is an appli-
cation of the perspective taken earlier by Mersenne
and Gassendi. Again, our knowledge of bodies is limited
to an acquaintance with their secondary qualities, not
their real natures. Shapes, colors, tactile qualities, and
the like can be observed but “their inward substances
are not to be known either by our senses or by any
reflex act of our minds” (Newton, Mathematical Princi-
..., p. 546). The world of primary qualities is
beyond our grasp, though one can formulate an hy-
pothesis about them. Newton's hypothesis is not unlike
the atomism of Boyle and Gassendi.

Newton's predecessors and contemporaries in the
Royal Society had delineated with care several levels
of certainty, each corresponding to a particular kind
of evidence. At the summit was usually the certainty
of immediate sense perception or that of mathematics
or metaphysics. Next were the several degrees of mor-
ally certain propositions, which class usually contained
those of science and religion. At the bottom was mere
probability. Boyle, for example, works out such an
hierarchical view. One can never be completely certain
of the conclusions of the sciences because of the limita-


tions of our abilities and because of the possibility that
God may change the course of nature (as occurs in
miracles) at the very moment we formulate laws. Sci-
entific conclusions for Boyle give rise to physical cer-
tainty when all conditions are at their best, but this
is an ideal seldom achieved. Generally, such conclu-
sions are only morally certain. Both of these kinds of
certainty, he says, are “but an inferior kind or degree
of certainty.” He regards the attempt of Descartes and
other physicists, for example, to prove that certain
comets are not meteors on the ground and that their
parallax is less than that of the moon, to have only
a moral certainty for they have not themselves made
the relevant observations.

Newton is much less explicit in his presentation,
distinguishing only mathematical propositions from
those that are physically certain, and pointing out that
scientific statements are not mathematically certain.
What the scientist regards as true may in fact turn
out to be false since his conclusions are based upon
merely human principles. Science cannot provide an
explanatory account of the real structure of nature but
must limit itself to a descriptive account of what is
observed to occur, and its conclusions are less than
absolutely or demonstratively certain. In his contro-
versy with Robert Hooke concerning the nature of light
he admits his conclusions are only physically certain
because of limitations in the evidence. One begins with
evidence that can be expressed in mathematical for-
mulae and from these deduces the existence of other
phenomena to be observed. The conclusions of one's
inferences are no more certain than the premisses from
which one begins; if these are inductive generalizations
one's conclusions have no greater certainty.

Newton's account of what can be known is applied
by him to religion as well as to science. One of the
features of the movement in English thought that we
have been tracing concerns the close relation between
evidence for religious and for scientific propositions.
In Chillingworth's writings references to scientific
knowledge occurred only incidentally; in Wilkins'
writings such references were of fundamental impor-
tance. The point was often made that the canons of
evidence which held for one also held for the other.
In Newton's thought the relation between religion and
science also plays a significant role. Indeed, for him
the pursuit of scientific truth has as one of its ultimate
justifications insight into religious truth. The first of
his letters to Richard Bentley begins with the comment
that the complexity of nature can be accounted for
only by reference to a deity with mathematical in-
genuity. In one of the Scholia to the Mathematical
Principles of Natural Philosophy
(1687) he notes, “And
so much concerning God, to discourse of whom from
the appearances of things, does certainly belong to
Natural Philosophy” (Newton, p. 546). Newton's point
seems to be that if the evidence for scientific and
religious propositions is of the same kind, then the
certainty for each will be the same also.

Such then is the reply of the seventeenth-century
mind to the crisis of skepticism. Two major alternatives
are worked out: the dogmatism of Bacon and Descartes
on the one side and the constructive skepticism of
Mersenne, Gassendi, Chillingworth, and the early
members of the Royal Society of London on the
other—a view which, after the middle of the eight-
eenth century, became the accepted account of cer-
tainty in all domains of knowledge. In the final decade
of the seventeenth century John Locke formulated an
account of the extent and certainty of knowledge
which articulates for all knowledge—not merely sci-
entific and religious knowledge—the constructive
skepticism we have been examining. But Locke's the-
ory of the certainty of knowledge also included some
elements of the Cartesian epistemology, a blending
which gave rise to a new series of problems for eight-
eenth-century thought.


Francis Bacon, Novum Organum, in The Works of Francis
eds. J. Spedding, R. L. Ellis, and D. D. Heath, 15
vols. (Boston, 1861). William Chillingworth, The Works of
William Chillingworth, containing his book The Religion of
Protestants, a Safe Way to Salvation
(Philadelphia, 1844).
John Dewey, The Quest for Certainty (New York, 1960,
reprint). René Descartes, The Philosophical Works of Des-
eds. E. S. Haldane and G. R. T. Ross, 2 vols.
(New York, 1955). John Locke, Essay Concerning Human
(1690; London and New York, 1924; many
reprints). Marin Mersenne, La Vérité des sciences contre les
sceptiques ou pyrrhoniens
(Paris, 1623); idem, Les Questions
théologiques, physiques, morales et mathématiques
1634). Isaac Newton, Mathematical Principles of Natural
Philosophy and his System of the World,
trans. A. Motte
(1729); rev. by F. Cajori (Berkeley; 1946). The Philosophical
Transactions of the Royal Society of London,
eds. C. Hutton,
G. Shaw, and R. Pearson, 18 vols. (London, 1809). Richard
H. Popkin, The History of Scepticism from Erasmus to
(New York, 1961). Henry G. Van Leeuwen, The
Problem of Certainty in English Thought 1630-1690
Hague, 1963). John Wilkins, Of the Principles and Duties
of Natural Religion
(London, 1699).


[See also Appearance and Reality; Axiomatization; Baco-
nianism; Causation; Certainty since the Seventeenth Cen-
Faith; Indeterminacy; Necessity; Newton on Method;
Reformation; Sin and Salvation; Skepticism.]