Reason. The Enlightenment has frequently been
arraigned for its overemphasis on abstract reason and
its neglect of imagination. Its representative thinkers
break with the rationalism and the esprit de système
of those who precede them. Locke's philosophy is their
philosophical bible, just as the detractors of the
Enlightenment later agree with de Maistre's verdict
that “philosophy begins with contempt for Locke.”
“The word reason... has different significations:
sometimes it is taken for true and clear principles;
sometimes for clear and fair deductions from those
principles; and sometimes for the cause, and particu-
larly the final cause. But the consideration I shall have
of it here is a signification different from all these...”
(Locke, An Essay concerning Human Understanding
[1690], Book IV, Ch. XVII, Para. 1). Locke's concern
is with “the original certainty, evidence and extent of
human knowledge, together with the grounds and
degrees of Belief, Opinion and Assent,” in short, with
reasoning or the discursive faculty, with proof, classifi-
cation, and deduction. There are marginal intimations
of the power of reason to provide a Baconian art of
discovery (ars inveniendi). On the whole, however,
Locke dwells on the limits of reason:
It is of great use to the sailor to know the length of his
line, though he cannot with it fathom all the depths of the
ocean. It is well he knows that it is long enough to reach
the bottom at such places as are necessary to direct his
voyage, and caution him against running upon shoals that
may ruin him. Our business here is not to know all things,
but those which concern our conduct. If we can find out
those measures whereby a rational creature, put in that state
which man is in in this world, may and ought to govern
his opinions, and actions depending thereon, we need not
be troubled that some other things escape our knowledge
(Book I, Ch. I, Para. 6).
Locke's investigation of human understanding is thus
part of the science of human nature which comes to
characterize the Enlightenment: It serves as the basis
for practical conduct; and though the formulation here
given seems to point to individual conduct, in practice,
because of the weaknesses inherent in individual
reasoning, it points to the science of the legislator as
the only area in which contriving and reforming man
is not necessarily out of his depth.
There is another, epistemological aspect to Locke's
philosophy which expresses itself in his idealism and
sensationalism, an aspect which interests professional
philosophers, and which was to make a great impact
on the theory of art and nineteenth-century materi-
alism (beginning with eighteenth-century sensa-
tionalists like Condillac and La Mettrie). However, for
the social philosophers of the Enlightenment, the
discovery that moral and material qualities are “not
qualities in objects, but perceptions of the mind...
has little or no influence on practice” (Hume, Treatise,
Book III, Part I, Sec. I). What counts, is the rejection
of metaphysical first causes which results in the setting
free of the “plain, historical method” of experience,
observation, and experiment.
This consideration applies also to Hume's skeptical
view of the limitations of reason. “Reason is nothing
but a wonderful and unintelligible instinct... this
instinct... arises from past observation and experi-
ence.... Reason is the discovery of truth and false-
hood... our passions, volitions and actions...
[cannot] be pronounced either true or false... reason
... can have an influence only after two ways: either
when it excites a passion, by informing us of the exist-
ence of something which is a proper object of it; or
when it discovers the connection of causes and
effects.... All knowledge resolves itself into proba-
bility...,” that is, the experience of many, of the
past, and of Trial and Error. Though “understanding,
when it acts alone, and according to its most general
principles, entirely subverts itself,” yet “when reason
is lively and mixes itself with some propensity it ought
to be assented to” (Hume, Treatise, Book I, Part III,
Sec. XVI; Book III, Part I, Sec. I; Book I, Part IV,
Sec. VII). For the alternative to reason is imagination
which, when acting unchecked by reason, leads into
superstition, illusion, and fanaticism. Imagination
controlled by reason, the creative human nature
protected from its destructive propensities by the
legislator and by education—this is the gist of the
theory of knowledge underlying the quest of the good
society of the reformers in the Enlightenment.
The awareness of the limitations of the human un-
derstanding rather than its overestimation determines
also the attitude of the Encyclopédie. “All certitude
which is not mathematical demonstration, is only ex-
treme probability. There is no other historical certi-
tude” (article “Histoire”). D'Alembert in the “Discours
préliminaire” of the Encyclopédie denigrates Descartes'
“believing he could explain everything,” and extols
Newton's insistence that scientific knowledge is merely
provisional, and that conjectures and hypotheses must
be presented as such and subjected to tests. He disap-
proves the application of logic and the “spirit of dis-
cussion” to the fields of literature and art because “the
passions and tastes have their own sort of logic”
(specifically Pascal's logique du coeur). The dissection
of the psychology of love (by Marivaux, Prévost, and
others) has ushered in a “species of the metaphysics
of the heart,” and “this 'anatomy of the soul' has even
slipped into our conversations; people make disser-
tations, they no longer converse; and our societies have
lost their principal ornaments—warmth and gaiety”
(ibid.). Diderot contrasts reason “coldly perceived”
unfavorably with the “brilliant and sublime” imagina-
tion: “Locke has seen, Shaftesbury has created” (article
“Génie”). He calls mathematicians “bad metaphysi-
cians... bad actors... bad politicians... such things
cannot be expressed in terms of X and Y. They depend
on a judicious observation of the intricate flow of life”
(undated letter, in Lettres à Sophie Volland, Paris
[1930], III, 279).
Whatever their weaknesses, the thinkers of the
Enlightenment pondered the problem of knowledge
more seriously than the thinkers of possibly any other
period. The customary strictures of their work are
largely derived from nonrational “flights of the imagi-
nation” and from the wish to defend old bastions and
temples. Far from being an abstract rationalist, Diderot
goes rather to the extreme of spontaneous, personal
knowledge. In the article “Éclecticisme” in the
Encyclopédie he foreshadows “the end of all schools”
of modern philosophy. Prejudice, tradition, antiquity,
and public opinion must be subjected by the philoso-
pher to a rational analysis and experience, “peculiarly
and personally his own.” This view has been said (by
Paul Hazard, la Pensée européenne au XVIIIe siècle,
Paris [1946], II, 48) to blink at the problem of solipsism.
However, the article as a whole makes it clear that
eclecticism requires both imaginative genius, the gift
to combine and explain, and the ability to gather evi-
dence and to put facts to the test; only he who com-
bines (objective) experimental and (subjective) system-
atic eclecticism, like Democritus, Aristotle, and Bacon,
may claim to be a truly eclectic philosopher in
Diderot's sense.
