Routledge History of Philosophy Volume IV

were mathematical in character. Of the seventeenth-century rationalists, some
played an important part in the new science. Descartes was philosopher,
mathematician and scientist; so, too, was Leibniz. Spinoza and Malebranche, for
their part, made no serious contribution to science or mathematics, but were well
informed about them. The question is how the seventeenth-century rationalists
saw philosophy as related to the natural sciences. 30 For present-day philosophers
of science, the sciences stand in no need of justification by philosophy, the
business of the philosopher being exclusively one of analysis: the clarification of
the nature of scientific propositions and of the methods of science. But it is clear
that this was not Descartes’s attitude; his search for foundations included
a search for the foundations of science, 31 and it is generally held that this is true
of the other seventeenth-century rationalists. 32
For the seventeenth-century rationalists, the foundations that they sought could
be discovered only by a priori reasoning. Perhaps the clearest arguments for this
thesis are provided by Descartes’s account of systematic doubt in the
Meditations, from which it emerges that he regards as known only those
propositions whose truth cannot be doubted, and also takes the view that such
propositions cannot be empirical. There would be general agreement that there is
such knowledge of the truths of logic and of mathematics; but Descartes argued
that these truths are only hypothetical, stating that if, for example, there is such a
figure as a triangle, then its interior angles must equal two right angles. 33 What
distinguishes the rationalists is their view that there are existential propositions
whose truth can be known a priori. Mathematics, although concerned only with
hypothetical truths, provided them with methods of procedure. 34 Roughly
speaking, what the rationalists tried to do was first of all to obtain a priori
knowledge of certain basic truths about what exists, and then to derive further
truths from these by means of pure reasoning.
As is well known, Descartes stated that the existential truth that he knew first
of all was the proposition that he existed as a thinking being, a proposition that
he could not doubt as long as he was actually thinking. But it is evident (and it
did not escape Descartes’s notice) that the truth of this proposition was in a way
far from fundamental, in that Descartes’s existence depended on that of many
other beings. Ultimately, the rationalists argued, it depended on the existence of
a supreme being. In a sense, therefore, the fundamental item of knowledge is the
knowledge that there must exist such a supreme being, or, as the rationalists said,
a ‘most perfect’ or a ‘necessary’ being. Belief in the existence of such a being
was not peculiar to the rationalists, but their arguments for its existence were
distinctive. These arguments had to be a priori, and the rationalists based them
on the concept of God. One argument offered by Descartes was that this concept
was such that only a God could have implanted it in us. Alternatively, Descartes
argued that the concept was such that one could not, without self-contradiction,
deny that God existed. This was the celebrated ‘ontological argument’, whose
soundness was accepted by Spinoza and Leibniz also. 35 INTRODUCTION 7