Descartes was, therefore, a bold exponent of what is known as the deductive or a priori method. Still, his approach must not be confused with the Scholasticism that, in philosopher Francis Bacon’s words, “flies from the senses and particulars to the most general axioms, and from these principles, the truth of which it takes for settled and immoveable, proceeds to judgment and to the discovery of middle axioms.” The fault of the Aristotelian syllogism, Descartes said, was that, although it helps people reason persuasively about things they already think they know, it is of no help in investigating the unknown. Thus, Descartes’s method was more radically a priori than Aristotle’s, which drew its premises from induction.
Turning his back on traditional logic and taking his cue from geometry, Descartes envisaged a chain of linear inferences that would progress from an initial truth so simple and obvious as to be self-evident to a second that would be seen at once to be included in the first, and thence to a third, and so forth. In practice, the problem would always be to find the simple truth to which the chain could be anchored; afterward, all that would be necessary would be to preserve the true order. Each particular truth along the way would be entirely obvious to anyone who understood what was being affirmed—just as in arithmetic, a child who understands a sum fathoms everything that is within reach of the greatest genius who contemplates the same set of figures.
For convenience, Descartes summed up his principles in four rules:1. Never accept any idea as true that is not so clearly and distinctly true as to be beyond all possibility of being doubted. 2. Divide each complex question into simple ones. 3. Order thoughts from the simplest to the most complex. 4. Review the series of inferences to make sure there are no breaks or false links in the chain.
If these rules were rigorously followed, an obscure matter, such as the function of the lungs in the body, would be rendered perfectly intelligible. Such, at least, was the promise that inspired the youthful Descartes and launched him on his great career.