A Foundational Scepticism

René Descartes took up Galileo Galilei’s kinematic science and his foundational method in his Discourse on Method. He challenged ancient forms of skepticism with a simple argument. Since we are rational animals, i.e., able to distinguish true from false judgments, we must, therefore, have a natural ability to tell some statements as clearly true. We can and must derive principles of philosophy deductively from such clear truths, committing no error. Our new science will be infallibly true. In applying the method, he used a reductio ad absurdum argument. He began with an undoubtedly known premise, showing that I cannot doubt everything. When I try, I cannot doubt that I exist. From “I exist” as an imperfect being, he derived that a perfect God exists, from whose invariable nature he derived the basic principles of motion, which he published later in his The Principles of Philosophy. Some of these principles (of inertia and force) became fundamental laws of motion in physics. But he admitted in the sixth part of the Discourse a fundamental limitation of his method. Though certainly known, he claimed, the universal principles cannot be applied to particulars in the world without assuming hypothetical models. The Cartesian method’s main weakness was its reliance on fictional hypotheses as models of reality when compared with the Baconian that did not rely on fiction.