In the NOVA program, Newton's Dark Secrets, it is claimed that Newton's work in physics was informed by his work in alchemy. For example, it seems that he thought of the force of gravity as an active principle, such as the one thought to be in operation in the so-called "vegetation of metals."
Is alchemy a pseudoscience? If so, why? What makes it different from chemistry?
Also, what can we learn from Newton's case about the nature of scientific discovery? Are there factors involved in scientific discovery that are not purely rational or logical? If so, what are those factors?