Rationalism existed as a movement during the 17th and 18th centuries. The main three names usually mentioned in connection with it are:

• Rene Descartes
• Baruch Spinoza
• Godfried Willhelm Leibniz

The French philosopher Decartes (1596-1650) is considered the father of modern philosophy. His main work, The Meditations, represents his attempt to establish a firm rational foundation for knowledge. To do this Descartes employed a method of doubt, whereby everything that he believed was called into question. Through this method Descartes claimed to have discovered the only undeniable truth: "I think, therefore I am". This allowed him, by combining this with other arguments - such as that for the existence of God, to guarantee that certain types of knowledge gained in certain ways was certain.

He is also famous for his views on mind and body, whereby he claims that the two are distinct and of different natures (this is called Cartesian Dualism).

Principal works:

- Discourse on Method

- Meditations on the First Philosophy

- Passions of the Soul

- Principles of Philosophy

The Dutch philosopher Spinoza (1632-1677), influenced by Descartes, developed a system within which he considered the problems involved in Descartes' philosophy could be overcome. For Spinonza, God was the identical with the world, leading many to view him as a pantheist (someone who believes that God is nature).

Spinoza's solution to the mind-body problem was to argue that mental and physical substance were in fact modes of the same substance (ie. God). In this sense, he was a monist (someone who believes in one substance) as opposed to a dualist like Descartes (someone who believes in two).

Principal works:

- Short Treatise Concerning God, Man and His Happiness

- Theological-Political Treatise

- Ethics Demonstrated Through the Method of Geometry (more commonly called simply the Ethics)

The German philosopher Leibniz (1646-1716) opposed both Descartes and Spinoza by arguing that the world was composed of an infinite number of substances (instead of one or two) which he called Monads. Leibniz's main contribution to philosophy is his idea that we live in the best of all possible worlds. This is an attempt to reconcile the idea that everything is pre-determined from the beginning of time by God, but also that human beings have free will. He held this position by arguing that although it was possible that the world could be different, it was not actually the case.

Leibniz also contributed to mathematics and is credited with the invention of the infinitesimal calculus.

Principal works:

- Theodicy

- Discourse on Metaphysics

- The New System of Nature

- New Essays Concerning Human Understanding

- Monadology