Defining rationalism
We have all likely heard the suggestion to “be rational.” This usually means to think or act in a way that is guided by reason. In philosophy, rationalism unpacks the faculty of reason, building a school of thought around the intellectual power of the mind.
Rationalism is one distinctive kind of answer to the philosophical question: how do we have knowledge? The field of philosophy dedicated to this question is known as epistemology and rationalism is a key player in Western epistemological history. Rationalism holds that, not only does reason have the power to generate substantive knowledge of the world, it also has the capacity to reflect on itself. So, by the use of reason we can come to understand features of the world, and features of our own minds.
One common way to define rationalism is through one of the great debates of Western philosophy: rationalism vs. empiricism. Empiricism is the epistemological position that all knowledge comes from sensory experience. For the empiricist, the mind is an empty receptacle that is only given content through experience of the world. According to empiricism, the faculty of reason only functions to manipulate impressions that originate in sensory experience. Rationalism accords much greater power to the faculty of reason.
Two philosophical distinctions serve to highlight the contrast between empiricism and rationalism, and thereby to help us better characterize rationalism: the innate / acquired distinction and the a priori / a posteriori distinction. Innate knowledge is knowledge that we are born with, and acquired knowledge is knowledge that is gained through experience. Naturally, empiricists find all knowledge to be acquired while rationalists think that we are already born with extensive knowledge of reality in the form of logic and reason.
The second distinction, a priori / a posteriori concerns the ways that a proposition can be known. A proposition can be known to be true a priori if knowing it does not depend upon prior experience. A proposition is said to be a posteriori if it can only be known through experience. For example, “post boxes are red” can only be known on the basis of experiencing the color of post boxes. It is an a posteriori proposition. But “nothing is both red and not red” can be known to be true independently of experience, by the use of reason alone. Even if you have never had the visual experience of red, you can know this statement to be true.
Rationalists and empiricists differ significantly over how much can be known a priori. For the rationalist much knowledge can be gained this way; for the empiricist only certain, limited kinds of propositions can be known by reason alone. Consider this example: “Bachelors are unmarried men.” This is clearly true, and you can know it even if you’ve never met a bachelor. You can know it merely by reflecting on the meanings of the terms “bachelor” and “unmarried men”; it is made true by the meanings of the words involved. In philosophy, this type of statement — one that is true by virtue of the meanings of the words — is known as “analytic.” Now consider the proposition “space is infinite.” This statement cannot be known simply by reflecting on the meanings of the words “space” and “infinite.” The concept of space does not contain the concept of infinite. Such statements are said by philosophers to be “synthetic.” They are substantive statements about the world.
With these distinctions in mind, the difference between empiricism and rationalism can be concisely summed up: empiricists believe that there are no synthetic a priori propositions while rationalists believe that there can be many. In other words, rationalists believe that substantive claims about the world can be made through reason alone.
This raises the question: what kinds of substantive claims can be known a priori, or by reason alone? And, what is innate knowledge? We will, in part, come to see how rationalists believe that claims about the nature of time and space, about causation, about mathematics, about morality, and about the nature of the human mind can be made by these methods.
The contention of rationalism that knowledge originates in the mind itself has significant consequences. It provokes the question: how can reason alone generate substantive knowledge of the world? Charlie Huenemann outlines the way that rationalists understand this relationship in Understanding Rationalism (2014), stating,