Rationalism in Philosophy

Rationalism is the philosophical stance according to which reason is the ultimate source of human knowledge. It stands in contrast to empiricism, according to which the senses suffice in justifying knowledge.

In one form or another, rationalism features in most philosophical traditions. In the Western tradition, it boasts a long and distinguished list of followers, including Plato, Descartes, and Kant. Rationalism continues to be a major philosophical approach to decision-making today.

Descartes' Case for Rationalism

How do we come to know objects — through the senses or through reason? According to Descartes, the latter option is the correct one.

As an example of Descartes' approach to rationalism, consider polygons (i.e. closed, plane figures in geometry). How do we know that something is a triangle as opposed to a square? The senses may seem to play a key role in our understanding: we see that a figure has three sides or four sides. But now consider two polygons — one with a thousand sides and the other with a thousand and one sides. Which is which? In order to distinguish between the two, it will be necessary to count the sides — using reason to tell them apart.
For Descartes, reason is involved in all of our knowledge. This is because our understanding of objects is nuanced by reason. For example, how do you know that the person in the mirror is, in fact, yourself? How does each of us recognize the purpose or significance of objects such as pots, guns, or fences? How do we distinguish one similar object from another? Reason alone can explain such puzzles.

Using Rationalism as a Tool for Understanding Ourselves in the World

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.

• How do we know who and what we are?  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. 
• What is the nature of cause and effect? Rationalists claim that causal links are known through reason. The empiricist's response is that it is only because of habit that we come to be convinced that, say, fire is hot.
• How do we know which actions are ethically correct?  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. 

Of course, in a practical sense, it is almost impossible to separate rationalism from empiricism. We cannot make rational decisions without the information provided to us through our senses, nor can we make empirical decisions without considering their rational implications.