Rationalism
Philosophical usage

Rationalism is often contrasted with empiricism. Taken very broadly, these views are not mutually exclusive, since a philosopher can be both rationalist and empiricist. Taken to extremes, the empiricist view holds that all ideas come to us a posteriori, that is to say, through experience; either through the external senses or through such inner sensations as pain and gratification. The empiricist essentially believes that knowledge is based on or derived directly from experience. The rationalist believes we come to knowledge a priori – through the use of logic – and is thus independent of sensory experience. In other words, as Galen Strawson once wrote, "you can see that it is true just lying on your couch. You don't have to get up off your couch and go outside and examine the way things are in the physical world. You don't have to do any science."

Between both philosophies, the issue at hand is the fundamental source of human knowledge and the proper techniques for verifying what we think we know. Whereas both philosophies are under the umbrella of epistemology, their argument lies in the understanding of the warrant, which is under the wider epistemic umbrella of the theory of justification. Part of epistemology, this theory attempts to understand the justification of propositions and beliefs. Epistemologists are concerned with various epistemic features of belief, which include the ideas of justification, warrant, rationality, and probability. Of these four terms, the term that has been most widely used and discussed by the early 21st century is "warrant". Loosely speaking, justification is the reason that someone (probably) holds a belief.

If A makes a claim and then B casts doubt on it, A ' s next move would normally be to provide justification for the claim. The precise method one uses to provide justification is where the lines are drawn between rationalism and empiricism (among other philosophical views). Much of the debate in these fields are focused on analyzing the nature of knowledge and how it relates to connected notions such as truth, belief, and justification.

At its core, rationalism consists of three basic claims. For people to consider themselves rationalists, they must adopt at least one of these three claims: the intuition/deduction thesis, the innate knowledge thesis, or the innate concept thesis. In addition, a rationalist can choose to adopt the claim of Indispensability of Reason and or the claim of Superiority of Reason, although one can be a rationalist without adopting either thesis.

The indispensability of reason thesis: "The knowledge we gain in subject area, S, by intuition and deduction, as well as the ideas and instances of knowledge in S that are innate to us, could not have been gained by us through sense experience." In short, this thesis claims that experience cannot provide what we gain from reason.

The superiority of reason thesis: '"The knowledge we gain in subject area S by intuition and deduction or have innately is superior to any knowledge gained by sense experience". In other words, this thesis claims reason is superior to experience as a source for knowledge.

Rationalists often adopt similar stances on other aspects of philosophy. Most rationalists reject skepticism for the areas of knowledge they claim are knowable a priori. When you claim some truths are innately known to us, one must reject skepticism in relation to those truths. Especially for rationalists who adopt the Intuition/Deduction thesis, the idea of epistemic foundationalism tends to crop up. This is the view that we know some truths without basing our belief in them on any others and that we then use this foundational knowledge to know more truths.
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