Exactly so, said Benedict Spinoza (1632-1677). Spinoza was born into Jewish faith, but read Descartes and renounced his Judaism. He was expelled from his synagogue, worked as a lensmaker, and was despised for centuries as an atheist.

Spinoza reasoned that there is only one (rational) substance in all reality, and apparent differences are only apparent. So, what is this substance?

Godfrey Wilhelm Leibniz (1646-1716) had the answer: "monads." Leibniz was sure that reality is made of these tiny units. "Monads" contain life in themselves, and are all of the same shape and size. They are arranged and moved by the "Prime Monad"--God. Higher-order monads (like us) perceive more than we reflect; lower-order monads (like this paper) reflect more than they perceive.

Balderdash, said the rest of the philosophical world. And they were right. Spinoza and Leibniz show that the ultimate result of pure rationalism is not rational at all. In their insistence that reason is the only substance in the world, they are not reasonable.

And the philosophical pendulum swings in the other direction.