Rationalism is the philosophical stance according to which reason is the ultimate source of human knowledge. It rivals empiricism according to which the senses suffice in justifying knowledge. In a form or another, rationalism features in most philosophical tradition; in the Western one, it boasts a long and distinguished list of followers, including Plato, Descartes, and Kant.
The Case for Rationalism
How do we come to know objects, through the senses or through reason? Descartes brought some of the strongest arguments to believe that the latter option is the correct one. Consider polygons (i.e. closed, plane figures in geometry). How do we come to recognize features of polygons; for example, how do we know that something is a triangle? The senses here may seem to play a key role: we see that a figure has three sides. But, now imagine to have two figures in front of you, the first with a thousand sides and the other with a thousand and one sides. Which is which? Well, presumably the senses will not suffice in providing an answer to this question: you will need reasoning (e.g. counting) in order to tell them apart.
For Descartes, reason is involved in all of our knowledge. This is because the nuances of the objects we encounter are far more than we can detect by the senses alone. Consider looking at a person waving at you for ten seconds: what you see are literally hundreds of different images; how do you know that they belong to one and the same gesture? And how do you know they belong to one and the same person? Now suppose that the person you are looking at is yourself in the mirror: how do you know you are looking at one person?
Reason alone can explain puzzles such as the one above. Other authors offered different arguments, such as Plato’s allegory of the cave or Spinoza’s arguments for God’s existence in the Ethics.
The Self and Causation, and Ethical Normativity
Since the justification of knowledge occupies a central role in philosophical theorizing, it is typical to sort out philosophers on the basis of their stance with respect to the rationalist vs empiricist debate. Rationalism indeed characterizes a wide range of philosophical topics, three main ones being personal identity, the nature of causation, and the source of ethical normativity.
Consider the self and causation first. Rationalists typically claim that the self is known through a rational intuition, which is irreducible to any sensorial perception of ourselves; empiricists, on the other hand, reply that the unity of the self is illusory. With respect to causation, rationalists claim that causal links are known through reason, while empiricists reply that it is only because of habit that we come be convinced that – say – fire is hot.
Finally, what is it that makes a certain action the one that we ought, morally, to perform? Kant argued that the ethical worth of an action can be understood only from a rational perspective; ethical evaluation is a rational game in which one or more rational agents envisage their actions under hypothetical conditions – if all the time someone who were in those circumstances you are facing, that person were to act in the way you are thinking of acting, would that seem feasible?
More generally, Kant stands in a category of its own when it comes to rationalism. His distinction between a priori and a posteriori judgments, indeed, can be seen as a modern way of restating the opposition between judgments that would be accepted (a posteriori) and those that would be off-limits (a priori) for an empiricist.
It should be noted, however, that not all rationalists defend analogous positions across the board. For example, Spinoza, Leibniz, and Kant, despite being all rationalists, defended vastly different views in ethics.
Further Online Readings and Sources
"Rationalism vs. Empiricism" at the Stanford Encyclopedia of Philosophy