Descartes: A New Approach

The first great philosopher of the modern era was René Descartes, whose new approach won him recognition as the progenitor of modern philosophy. Descartes's pursuit of mathematical and scientific truth soon led to a profound rejection of the scholastic tradition in which he had been educated. Much of his work was concerned with the provision of a secure foundation for the advancement of human knowledge through the natural sciences. Fearing the condemnation of the church, however, Descartes was rightly cautious about publicly expressing the full measure of his radical views. The philosophical writings for which he is remembered are therefore extremely circumspect in their treatment of controversial issues.


After years of work in private, Descartes finally published a preliminary statement of his views in the Discourse on the Method of Rightly Conducting the Reason (1637). Since mathematics has genuinely achieved the certainty for which human thinkers yearn, he argued, we rightly turn to mathematical reasoning as a model for progress in human knowledge more generally. Expressing perfect confidence in the capacity of human reason to achieve knowledge, Descartes proposed an intellectual process no less unsettling than the architectural destruction and rebuilding of an entire town. In order to be absolutely sure that we accept only what is genuinely certain, we must first deliberately renounce all of the firmly held but questionable beliefs we have previously acquired by experience and education.
The progress and certainty of mathematical knowledge, Descartes supposed, provide an emulable model for a similarly productive philosophical method, characterized by four simple rules:

1. Accept as true only what is indubitable.
2. Divide every question into manageable parts.
3. Begin with the simplest issues and ascend to the more complex.
4. Review frequently enough to retain the whole argument at once.

This quasi-mathematical procedure for the achievement of knowledge is typical of a rationalistic approach to epistemology. While engaged in such a comprehensive revision of our beliefs, Descartes supposed it prudent to adhere to a modest, conventional way of life that provides a secure and comfortable environment in which to pursue serious study. The stoic underpinnings of this "provisional morality" are evident in the emphasis on changing oneself to fit the world. Its general importance as an avenue to the contemplative life, however, is more general. Great intellectual upheavals can best be undertaken during relatively calm and stable periods of life.

Anticipated Results

In this context, Descartes offered a brief description of his own experience with the proper approach to knowledge. Begin by renouncing any belief that can be doubted, including especially the testimony of the senses; then use the perfect certainty of one's own existence, which survives this doubt, as the foundation for a demonstration of the providential reliability of one's faculties generally. Significant knowledge of the world, Descartes supposed, can be achieved only by following this epistemological method, the rationalism of relying on a mathematical model and eliminating the distraction of sensory information in order to pursue the demonstrations of pure reason.
Later sections of the Discourse (along with the supplementary scientific essays with which it was published) trace some of the more significant consequences of following the Cartesian method in philosophy. His mechanistic inclinations emerge clearly in these sections, with frequent reminders of the success of physical explanations of complex phenomena. Non-human animals, on Descartes's view, are complex organic machines, all of whose actions can be fully explained without any reference to the operation of mind in thinking.
In fact, Descartes declared, most of human behavior, like that of animals, is susceptible to simple mechanistic explanation. Cleverly designed automata could successfully mimic nearly all of what we do. Thus, Descartes argued, it is only the general ability to adapt to widely varying circumstances—and, in particular, the capacity to respond creatively in the use of language—that provides a sure test for the presence of an immaterial soul associated with the normal human body.
But Descartes supposed that no matter how human-like an animal or machine could be made to appear in its form or operations, it would always be possible to distinguish it from a real human being by two functional criteria. Although an animal or machine may be capable of performing any one activity as well as (or even better than) we can, he argued, each human being is capable of a greater variety of different activities than could be performed by anything lacking a soul. In a special instance of this general point, Descartes held that although an animal or machine might be made to utter sounds resembling human speech in response to specific stimuli, only an immaterial thinking substance could engage in the creative use of language required for responding appropriately to any unexpected circumstances. My puppy is a loyal companion, and my computer is a powerful instrument, but neither of them can engage in a decent conversation. (This criterion anticipated the more formal requirements of the Turing test.)