What was the principal difference between René Descartes' method for finding the truth and Francis Bacon's?

Descartes' method was rationalism, working deductively from principles inside the mind. Bacon's method was empiricism, working inductively from the evidence of experience.

Explanation:

Francis Bacon (1561 - 1626) recommended the sort of thing we now call the scientific method. His approach is known as empiricism, meaning that truth is arrived at by experimentation and the observations of experience. He even went so far as to say that the best form of science does not start with a hypothesis, because then the researcher would be biased toward wanting to prove his hypothesis correct. Bacon's recommendation was to start purely by observing all available data and sifting through it, letting a hypothesis present itself from the observed facts. An example of this would be the way a crime scene investigator processes a crime scene - - simply documenting everything about the scene and working through the evidence before arriving at a theory of what happened.

René Descartes (1596 - 1650) took a different approach, which is called rationalism. Descartes proposed that we can't be sure of truth by relying on our senses and observed experiences, because our senses can be deceived. We also could be dreaming or hallucinating. So he wanted a path to truth that started only with what we had before observing anything else - - which meant starting inside his own mind. "I think, therefore I am," was Descartes' famous statement that asserted that, at the very least, he knew he existed because he was thinking. Even if he was dreaming or hallucinating, he (as a thinking thing of some sort) had to exist in order to be doing that. From there he extrapolated to show that our experience of our own bodies and the physical world could be trusted, at least generally so, as we pursue knowledge. It would take a lot to explain his path of thought in his book, Discourse on the Method of Rightly Conducting One's Reason and of Seeking Truth in the Sciences (1637). This has already been a long enough explanation, though, so I'll stop here!