Routledge History of Philosophy Volume IV

DESCARTES: METHODOLOGY 177
In every train of reasoning it is merely by comparison that we attain to a
precise knowledge of the truth. Here is an example: all A is B, all B is C,
therefore all A is C. Here we compare with one another what we are
searching for and what we are given, viz. A and C, in respect of the fact that
each is B, and so on. But, as we have pointed out on a number of occasions,
because the forms of the syllogism are of no aid in perceiving the truth
about things, it will be better for the reader to reject them altogether and to
conceive that all knowledge whatsoever, other than that which consists in
the simple and pure intuition of single independent objects, is a matter of
the comparison of two things or more with each other. In fact practically
the whole task set the human reason consists in preparing for this operation;
for when it is open and simple, we need no aid from art, but are bound to
rely upon the light of nature alone, in beholding the truth which
comparison gives us. 40
The difference between intuition and deduction lies in the fact that whereas the
latter consists in grasping the relations between a number of propositions,
intuition consists in grasping a necessary connection between two propositions.
But in the limiting case, deduction reduces to intuition: we run through the
deduction so quickly that we no longer have to rely on memory, with the result
that we grasp the whole in a single intuition at a single time. The core of
Descartes’s position is that by compacting inferential steps until we come to a
direct comparison between premises and conclusion we put ourselves in a
position where we are able to have a clear and distinct idea of the connection,
and this provides us with a guarantee of certainty.
What is at issue here is the question of the justification of deduction, but we
must be careful to separate out two different kinds of demand for justification.
The first is a demand that deductive inference show itself to be productive of new
knowledge, that it result in episte-mic advance. The second is a question about
whether deductive inference can be further analysed or explained: it is a question
about the justification of deduction, but not one which refers us to its epistemic
worth for, as Dummett has rightly pointed out, our aim ‘is not to persuade
anyone, not even ourselves, to employ deductive arguments: it is to find a
satisfactory explanation for the role of such arguments in our use of language.’ 41
Now these two kinds of question were not always clearly distinguished in
Descartes’s time, and it was a prevalent assumption in the seventeenth century that
syllogistic, in both its logical and its heuristic aspects, could be justified if and
only if it could show its epistemic worth. But the basis for the distinction was
certainly there, and while the questions are related in Descartes, we can find sets
of considerations much more relevant to the one than the other.
The first question, that of epistemic informativeness, concerns the use of
formalized deductive arguments, especially the syllogism, in the discovery of
new results in natural philosophy. Earlier, we looked very briefly at how
Aristotle tried to deal with this question, by distinguishing two different forms of