Rene Descartes is frequently considered the first modern philosopher. His first publication, Discourse on Method (1637), was the touchstone of the scientific method. A response to the lack of clarity he saw in the world of science, Discourse describes how scientific study should be prosecuted so as to achieve the utmost clarity, by using deductive reasoning to test hypotheses. Descartes explained that the test of an alleged truth is the clarity with which it may be apprehended, or proven. "I think, therefore I am," (cogito ergo sum) is Descartes' famous example of the most clearly apprehended truth. In effect, the evidence of thought proves the hypothesis of existence.

Descartes dabbled extensively in the study of cosmology and the nature of matter, developing theories on the make up of matter and the formulation and operation of heavenly bodies. Though Descartes' astronomical explanation failed to account for many observed phenomena, his great prestige propelled his theory into fashion among the educated elite intellectuals of Europe. Descartes was even about to publish a book on cosmology, entitled The World, in 1653, when he heard of Galileo Galilei's condemnation by the Church and thought better of it. Descartes tried to apply his physical theories and expand upon them in his works on human anatomy, which, though pioneering in some respects, were largely erroneous. He further wrote about the spiritual nature of man and theorized about the existence of the soul. The Cartesian philosophy (derived from his name, Descartes) won many followers during the seventeenth century.

Francis Bacon, also called Lord Verulam, was somewhat less renowned and less successful than Descartes, but nevertheless highly influential. Bacon advocated the collection of all possible facts and phenomena and the processing of these through a sort of automatic logical mill. Bacon warned scientists against four famous false notions, called Idols.

1. Idols of the Tribe were fallacies in humankind, most notably man's proneness to believe that nature was ordered to a higher degree than it actually was.

2. Idols of the Cave were misconceptions inherent in individuals' thoughts, spawned by private prejudices.

3. Idols of the Marketplace were errors that arose from received systems of thought.

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