11. Extensio. Our term extension comes from the
late Latin term extensio—itself derived from the clas-
sical verb extendere—and it became a philosophical
term in the Middle Ages. The exact philosophical status
in the Middle Ages is not easy to determine, and this
is due to a general difficulty which is tellingly presented
in the following passage from a book on Duns Scotus:
Thus the nature of Space is discussed with reference to
transubstantiation and the nature of angelic operation,
while that of Time, though treated more thoroughly and
at greater length in the De rerum principio, is once more
mooted in the commentaries on the Sentences in connection
with the angelic experience. Nor is this all. Our difficulties
are increased by the fact that the scholastic terminology
is almost impossible to translate exactly. For spatial relations
are expressed in terms of accidens, respectus and funda-
mentum, all logical rather than mathematical symbols. In
fact, the entire physics of the medieval world reflects this
logical view of things so strange to our modern scientific
modes of thinking
(C. R. S. Harris, 2, 173).
In philosophy after 1600, extension leapt into
prominence when Descartes used it, together with the
equivalent étendue, in his Philosophical Principles. Oc-
casionally, Descartes writes espace for it, but only
informally, because formally espace is something else
for him. In fact, in La Géométrie Descartes introduces
an espace (qui a trois dimensions) as an operational
background space for coordinate geometry in mathe-
matics, and this is the true role of espace in the thinking
of Descartes. Extension however is for him something
conceptually different, namely the space of physics and
of the universe. In this role, extension is coextensive
with matter, certainly with matière subtile, and it is
the carrier of Cartesian vortices (Hesse, pp. 102-08).
After Descartes, extension gradually diminished in
importance, or at least in prominence. In Spinoza's
Ethics it is “identified” with Spinoza's God (Wolfson,
Ch. VII), and it then occurs in Leibniz' reaction to
Spinoza (Leibniz Selections, pp. 485ff.). It still has a
standing in the theory of perception of George
Berkeley, Bishop of Cloyne (Jammer, p. 133) but after
that it began to be a philosophical term of second rank.
But the “subtle matter” of Descartes, which filled
his extension, maintained itself longer, although it had
already had a long career, starting out with the role
of Aristotle's body (soma) which filled his topos. Philos-
ophers of the eighteenth century showed signs of tiring
of this “subtle matter,” but, unperturbed by this, it
somehow managed to become the front-page aether
of James Clerk Maxwell in the nineteenth century
(Whittaker, Vol. 1, Ch. IX). Only the early twentieth
century finally sent it into retirement, but it took an
Albert Einstein to bring this about.
Instead of aether there are nowadays various “fields;”
gravitational field, electromagnetic field, fields of vari-
ous de Broglie waves. The fields are dual to particles
of matter or energy, and energy is equivalent with
mass, so that a return to a “subtle matter” has been
effected. Physics has but a limited budget of ideas of
cognition with which to operate, and the same ideas
are likely to return every so often for reassignment.
