Routledge History of Philosophy Volume IV

188 RENAISSANCE AND SEVENTEENTH-CENTURY RATIONALISM
the whole system. Complete certainty could be attained only by ‘demolishing
everything completely and starting again right from the foundations’. 4 It is this
‘foundational’ project that forms the core of Cartesian metaphysics.
In addition to his celebrated architectural metaphor of demolishing and
rebuilding, Descartes also made use of an organic simile to explain the
importance of metaphysics: ‘The whole of philosophy is like a tree: the roots are
metaphysics, the trunk is physics and the branches emerging from the trunk are
all the other sciences.’ 5 The simile is sometimes interpreted to mean that
metaphysics is, for Descartes, the most important part of philosophy; but this is
in some respects misleading. Descartes himself goes on to observe that ‘it is not
the roots or the trunk of a tree from which one gathers fruit, but only the
branches’, and he evidently saw the principal goal of his system as that of
yielding practical benefits for mankind: in place of the ‘speculative philosophy
taught in the schools’ he aimed to develop a ‘practical philosophy’ which would
be ‘useful in life’ and ultimately make us ‘lords and masters of nature’. 6
Metaphysics was in this sense a means to an end, for Descartes, rather than an
end in itself; he had no patience with abstract speculation for its own sake, and
frequently told questioners and correspondents not to become bogged down in
metaphysical inquiries. 7 Nevertheless, Descartes believed that at least once in a
lifetime (semel in vita) 8 anyone pretending to construct a reliable system of
knowledge would have to engage in metaphysical inquiries: without such
inquiries, there could be no guarantee of the stability of the rest of the system.
Indeed (and the tree simile is again illuminating here), Descartes regarded the
whole of human knowledge as a quasi-organic unity: in place of the scholastic
conception of knowledge (ultimately derived from Aristotle) as an amalgam of
separate disciplines, each with its own standards of precision and methods of
inquiry, Descartes (reverting to an older Platonic idea) saw all truths as
essentially interconnected. We need to grasp, he wrote in an early notebook, that
all the sciences are ‘linked together’ like a series of numbers; 9 later he developed
the idea further: ‘those long chains of very simple and easy reasonings which
geometers customarily use to arrive at their most difficult demonstrations gave me
occasion to suppose that all the items which fall within the scope of human
knowledge are interconnected in the same way.’ 10 Cartesian metaphysics
attempts to start from scratch and establish, once and for all, the philosophical basis
for these interconnections, aiming thereby to provide a kind of validation for the
system as a whole.
THE SIMPLE NATURES
From some standard accounts of Descartes’s life one might get the impression
that as a young man he was predominantly concerned with mathematical and
scientific issues, and that his metaphysical interests came later. It is certainly true
that mathematics was a major preoccupation of the young Descartes. Many of
the results later incorporated in his Geometry 11 were worked out during the