To be a rationalist is to adopt at least one of three claims. The Intuition/Deduction thesis concerns how we become warranted in believing propositions in a particular subject area.
The Intuition/Deduction Thesis: Some propositions in a particular subject area, S, are knowable by us by intuition alone; still others are knowable by being deduced from intuited propositions.
Intuition is a form of rational insight. Intellectually grasping a proposition, we just "see" it to be true in such a way as to form a true, warranted belief in it. Deduction is a process in which we derive conclusions from intuited premises through valid arguments, ones in which the conclusion must be true if the premises are true. We intuit, for example, that the number three is prime and that it is greater than two. We then deduce from this knowledge that there is a prime number greater than two. Intuition and deduction thus provide us with knowledge a priori, which is to say knowledge gained independently of sense experience.
We can generate different versions of the Intuition/Deduction thesis by substituting different subject areas for the variable ‘S’. Some rationalists take mathematics to be knowable by intuition and deduction. Some place ethical truths in this category. Some include metaphysical claims, such as that God exists, we have free will, and our mind and body are distinct substances. The more propositions rationalists include within the range of intuition and deduction, and the more controversial the truth of those propositions, the more radical their rationalism.
Rationalists also vary the strength of their view by adjusting their understanding of warrant. Some take warranted beliefs to be beyond even the slightest doubt and claim that intuition and deduction provide beliefs of this high epistemic status. Others interpret warrant more conservatively, say as belief beyond a reasonable doubt, and claim that intuition and deduction provide beliefs of that caliber.
Still another dimension of rationalism depends on how its proponents understand the connection between intuition, on the one hand, and truth, on the other. Some take intuition to be infallible, claiming that whatever we intuit must be true. Others allow for the possibility of false intuited propositions.
The second thesis associated with rationalism is the Innate Knowledge thesis.
The Innate Knowledge Thesis: We have knowledge of some truths in a particular subject area, S, as part of our rational nature.
Like the Intuition/Deduction thesis, the Innate Knowledge thesis asserts the existence of knowledge gained a priori, independently of experience. The difference between them rests in the accompanying understanding of how this a priori knowledge is gained. The Intuition/Deduction thesis cites intuition and subsequent deductive reasoning. The Innate Knowledge thesis offers our rational nature. Our innate knowledge is not learned through either sense experience or intuition and deduction. It is just part of our nature. Experiences may trigger a process by which we bring this knowledge to consciousness, but the experiences do not provide us with the knowledge itself. It has in some way been with us all along. According to some rationalists, we gained the knowledge in an earlier existence. According to others, God provided us with it at creation. Still others say it is part of our nature through natural selection.
We get different versions of the Innate Knowledge thesis by substituting different subject areas for the variable ‘S'. Once again, the more subjects included within the range of the thesis or the more controversial the claim to have knowledge in them, the more radical the form of rationalism. Stronger and weaker understandings of warrant yield stronger and weaker versions of the thesis as well..
The third important thesis of rationalism is the Innate Concept thesis.
The Innate Concept Thesis: We have some of the concepts we employ in a particular subject area, S, as part of our rational nature.
According to the Innate Concept thesis, some of our concepts are not gained from experience. They are part of our rational nature in such a way that, while sense experiences may trigger a process by which they are brought to consciousness, experience does not provide the concepts or determine the information they contain. Some claim that the Innate Concept thesis is entailed by the Innate Knowledge Thesis; a particular instance of knowledge can only be innate if the concepts that are contained in the known proposition are also innate. This is Locke's position (Essay Concerning Human Understanding, Book I, Chapter IV, Section 1, p. 91). Others, such as Carruthers, argue against this connection (Human Knowledge and Human Nature, pp. 53-54). The content and strength of the Innate Concept thesis varies with the concepts claimed to be innate. The more a concept seems removed from experience and the mental operations we can perform on what experience provides the more plausibly it may be claimed to be innate. Since we do not experience perfect triangles but do experience pains, our concept of the former is a more promising candidate than our concept of the latter for being innate.
The Intuition/Deduction thesis, the Innate Knowledge thesis, and the Innate Concept thesis are essential to rationalism: to be a rationalist is to adopt at least one of them. Two other closely related theses are generally adopted by rationalists, although one can certainly be a rationalist without adopting either of them. The first is that experience cannot provide what we gain from reason.
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.
The second is that reason is superior to experience as a source of knowledge.
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.
How reason is superior needs explanation, and rationalists have offered different accounts. One view, generally associated with Descartes (Rules for the Direction of our Native Intelligence, Rules II and III, pp.1-4), is that what we know a priori is certain, beyond even the slightest doubt, while what we believe, or even know, on the basis of sense experience is at least somewhat uncertain. Another view, generally associated with Plato (Republic 479e-484c), locates the superiority of a priori knowledge in the objects known. What we know by reason alone, a Platonic form, say, is superior in an important metaphysical way, e.g. unchanging, eternal, perfect, a higher degree of being, to what are aware of through sense experience.
Most forms of rationalism involve notable commitments to other philosophical positions. One is a commitment to the denial of scepticism for at least some area of knowledge. If we claim to know some truths by intuition or deduction or to have some innate knowledge, we obviously reject scepticism with regard to those truths. Rationalism in the form of the Intuition/Deduction thesis is also committed to epistemic foundationalism, 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.
Source: Markie, Peter, "Rationalism vs. Empiricism", The Stanford Encyclopedia of Philosophy (Fall 2008 Edition), Edward N. Zalta (ed.), URL =