The Rationalists were a movement of philosophers who emerged in the seventeenth century focusing on ‘a priori’ knowledge, which is knowledge attainable through reason alone, and without the need for sense information. This is in contrast with the Empiricists, who privileged sense perception. At the time, Rationalist thinking could have been quite dangerous due to laws based around strict Christianity throughout much of Europe at the time. Today, however, we are able to take reasoned thinking for granted. Well, at least some of us are!
Descartes, Leibniz and Spinoza are commonly taken to be the three main rationalist thinkers. The rationalists were inspired by the many advances in mathematics and the sciences during the Enlightenment, and attempted to build metaphysical systems of understanding in much the same way that mathematical or scientific systems are formed. That is, they gradually built up theories from principles of certainty through reason in the attempt to provide a coherent metaphysical explanation of the world.
For example, Descartes argued that one could not know objects purely through sense experience, but must use reason. This is because sense experience tells one how an object looks, but this will change, such as if the object melts. Descartes would say one uses reason to recognise that, for example, water and ice are the same thing. As a more contemporary Philosopher, Nigel Warburton points out this example “reveals Descartes’ rationalism,” and his belief that true understanding and knowledge must come from reason rather than the information from our senses.
The Rationalists were hugely influential in developing new methods of Philosophy.
Some critics argue that Rationalism assumes that reason can accomplish too much. For example, Empiricist Hume argued that the link between cause and effect must not be based upon reasoning but on experience of the effect resulting from the cause until the next time. It is possible A will not always result in B thus we must use experience to verify the outcome each time. Reason cannot be the only means of achieving knowledge when we gather so much via our senses.
However, one could argue that cause and effect cannot be discovered by sense experience alone. One must use reason to deduce that if A has a certain relationship with B, A caused B. Sense experience can only show that two events relate to each other in a certain way, not causation.
Further, Nietzsche rejected the Rationalist’s search for absolute truth based on reason, independent of perspective, on the ground that there is no evidence that this exists.