Routledge History of Philosophy Volume IV

RENAISSANCE AND SEVENTEENTH-CENTURY RATIONALISM 357
The thesis that only the presence of a soul can confer unity on a body, and thus
make it a genuine substance rather than an aggregate, obviously needs to be
justified. Leibniz is not totally forthcoming on this subject, but he does throw out
some suggestive hints which make it possible for us to see what he has in mind.
In correspondence with Arnauld he explains that the unity of an aggregate is a
matter of convention only. 14 The unity of a university department, for example,
is conventional in the sense that it depends on certain human interests; for
teaching purposes, let us say, it is convenient to group a Leibniz specialist with a
philosopher of language rather than with a seventeenth-century historian. But there
is no metaphysical fact of the matter which determines this classification. The
unity of a human body, however, is not at all like that. The fact that my hand and
foot belong together, but not my hand and the table in front of me, is determined
not by convention but by nature, or rather by the metaphysical truth that my soul
animates my body. I can, for example, feel pain in my hand and foot, but I
cannot feel pain in the table in front of me. Thus the presence of a soul provides
a wholly non-conventional basis for classifying some physical parts together.
Leibniz’s doctrine that organisms are true substances was the target of two
shrewd objections from Arnauld. In the first place, Arnauld objected that Leibniz
seemed to be smuggling in a merely stipulative definition of substance. As
Arnauld sees it, Leibniz redefines substance as that which has a true unity, and
on this basis he reaches the anti-Cartesian conclusion that no bodies except
organisms are substances. But in that case he has covertly abandoned the
traditional definition of substance as that which is neither a mode nor state; using
more Aristotelian language, we could restate Arnauld’s point by saying that
Leibniz has abandoned the definition of substance as an ultimate subject of
predication. 15 Leibniz is of course entitled to offer a stipulative definition of
substance if he chooses, but he is not entitled to switch back and forth between
such a definition and a more traditional one. Leibniz’s reply to this objection is
important: he answers Arnauld by saying that far from abandoning Aristotle’s
definition of substance he is simply drawing out a consequence of it: being a true
unity is implied by being an ultimate subject of predication. Indeed, the concepts
of a true unity and of an ultimate subject of predication are logically equivalent.
‘To be brief, I hold as axiomatic the identical proposition, which varies only in
emphasis: that what is not truly one entity is not truly one entity either. It has always
been thought that ‘one’ and ‘entity’ are interchangeable.’ 16
Arnauld also objected to Leibniz’s reintroduction of animal souls or substantial
forms. As a good Cartesian, Arnauld made the familiar points against this
doctrine; it is superfluous for the purposes of explaining animal behaviour, and it
raises embarrassing difficulties concerning the status of animal souls after the
destruction of their bodies. Arnauld cited the case of a worm both parts of which,
when cut in two, continue to move as before, and challenged Leibniz as to what
he would say about it. 17 The serious philosophical point behind Arnauld’s
raillery is that an animal is no more a genuine unity than a non-organic body
such as a table. Thus the chopping up of a worm is in principle no different from