Routledge History of Philosophy Volume IV

176 RENAISSANCE AND SEVENTEENTH-CENTURY RATIONALISM
the relation between his ‘method of discovery’ and his ‘method of presentation’,
but in an altogether deeper and more intractable question about how inference
can be informative. Inference is necessarily involved in every kind of scientific
enterprise, from logic and mathematics to natural philosophy, and the whole
point of these enterprises is to produce new knowledge, but the canonical form
of inference, for Descartes and all his predecessors, is deductive inference, and it
is a highly problematic question whether deductive inference can advance
knowledge.
The question became highlighted in the sixteenth century when there was
intense discussion of the Aristotelian distinction between knowledge how and
knowledge why, and the ways in which the latter could be achieved. Turnebus, 37
writing in 1565, tells us that the (Aristotelian) question of method was the most
discussed philosophical topic of the day. These debates were conducted in the
context of the theory of the syllogism, and although, with the demise of
syllogistic, the explicitly logical context is missing from seventeenth-century
discussions of method, there is always an undercurrent of logical questions.
Descartes raises the question of method in the context of considerations about the
nature of inference in the following way in Rule 4 of the Rules:
But if our method rightly explains how intellectual intuition should be
used, so as not to fall into error contrary to truth, and how one must find
deductive paths so that we might arrive at knowledge of all things, I cannot
see anything else is needed to make it complete; for I have already said
that the only way science is to be acquired is by intellectual intuition or
deduction. 38
Intellectual intuition is simply the grasp of a clear and distinct idea. But what is
deduction? In Rule 7 it is described in a way which makes one suspect that it is
not necessary in its own right:
Thus, if, for example, I have found out, by distinct mental operations, what
relation exists between magnitudes A and B, then what between B and C,
between C and D, and finally between D and E, that does not entail that I will
see what the relation is between A and E, nor can the truths previously
learned give me a precise idea of it unless I recall them all. To remedy this
I would run over them many times, by a continuous movement of the
imagination, in such a way that it has an intuition of each term at the same
time that it passes on to the others, and this I would do until I have learned
to pass from the first relation to the last so quickly that there was almost no
role left for memory and I seemed to have the whole before me at the same
time. 39
This suspicion is confirmed in Rule 14: