Any theory emphasizing reason or intuition (usually in contrast to the senses, or, in ethics, to feelings and emotions), whether as the basis for acquiring knowledge, or as the basis for justifying moral judgments.

In these uses it contrasts with empiricism, and has similar varieties. The a priori is to rationalism what the a posteriori (or empirical) is to empiricism. In ethics, rationalism tends to be classed with intuitionism (though different from it) in opposition to moral sense theories.

The late 19th and 20th centuries’ use of ‘rationalism’ to contrast with belief in religious revelation is not common in philosophy.

Also see: continental rationalists

W von Leyden, Seventeenth Century Metaphysics (1968), especially ch. 3

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.[2] 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.”[27] 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.

Theory of justification

The theory of justification is the part of epistemology that 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.

Thesis of rationalism

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 intuition/deduction thesis

Rationale: “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.”[28]

Generally speaking, intuition is a priori knowledge or experiential belief characterized by its immediacy; a form of rational insight. We simply “see” something in such a way as to give us a warranted belief. Beyond that, the nature of intuition is hotly debated.

In the same way, generally speaking, deduction is the process of reasoning from one or more general premises to reach a logically certain conclusion. Using valid arguments, we can deduce from intuited premises.

For example, when we combine both concepts, we can intuit 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. Thus, it can be said that intuition and deduction combined to provide us with a priori knowledge – we gained this knowledge independently of sense experience.

Empiricists such as David Hume have been willing to accept this thesis for describing the relationships among our own concepts.[28] In this sense, empiricists argue that we are allowed to intuit and deduce truths from knowledge that has been obtained a posteriori.

By injecting different subjects into the Intuition/Deduction thesis, we are able to generate different arguments. Most rationalists agree mathematics is knowable by applying the intuition and deduction. Some go further to include ethical truths into the category of things knowable by intuition and deduction. Furthermore, some rationalists also claim metaphysics is knowable in this thesis.

In addition to different subjects, rationalists sometimes vary the strength of their claims by adjusting their understanding of the warrant. Some rationalists understand warranted beliefs to be beyond even the slightest doubt; others are more conservative and understand the warrant to be belief beyond a reasonable doubt.

Rationalists also have different understanding and claims involving the connection between intuition and truth. Some rationalists claim that intuition is infallible and that anything we intuit to be true is as such. More contemporary rationalists accept that intuition is not always a source of certain knowledge – thus allowing for the possibility of a deceiver who might cause the rationalist to intuit a false proposition in the same way a third party could cause the rationalist to have perceptions of nonexistent objects.

Naturally, the more subjects the rationalists claim to be knowable by the Intuition/Deduction thesis, the more certain they are of their warranted beliefs, and the more strictly they adhere to the infallibility of intuition, the more controversial their truths or claims and the more radical their rationalism.[28]

To argue in favor of this thesis, Gottfried Wilhelm Leibniz, a prominent German philosopher, says, “The senses, although they are necessary for all our actual knowledge, are not sufficient to give us the whole of it, since the senses never give anything but instances, that is to say particular or individual truths. Now all the instances which confirm a general truth, however numerous they may be, are not sufficient to establish the universal necessity of this same truth, for it does not follow that what happened before will happen in the same way again. … From which it appears that necessary truths, such as we find in pure mathematics, and particularly in arithmetic and geometry, must have principles whose proof does not depend on instances, nor consequently on the testimony of the senses, although without the senses it would never have occurred to us to think of them…”[29]

The innate knowledge thesis

Rationale: “We have knowledge of some truths in a particular subject area, S, as part of our rational nature.”[30]

The Innate Knowledge thesis is similar to the Intuition/Deduction thesis in the regard that both theses claim knowledge is gained a priori. The two theses go their separate ways when describing how that knowledge is gained. As the name, and the rationale, suggests, the Innate Knowledge thesis claims knowledge is simply part of our rational nature. Experiences can trigger a process that allows this knowledge to come into our consciousness, but the experiences don’t provide us with the knowledge itself. The knowledge has been with us since the beginning and the experience simply brought into focus, in the same way a photographer can bring the background of a picture into focus by changing the aperture of the lens. The background was always there, just not in focus.

This thesis targets a problem with the nature of inquiry originally postulated by Plato in Meno. Here, Plato asks about inquiry; how do we gain knowledge of a theorem in geometry? We inquire into the matter. Yet, knowledge by inquiry seems impossible.[31] In other words, “If we already have the knowledge, there is no place for inquiry. If we lack the knowledge, we don’t know what we are seeking and cannot recognize it when we find it. Either way we cannot gain knowledge of the theorem by inquiry. Yet, we do know some theorems.”[30] The Innate Knowledge thesis offers a solution to this paradox. By claiming that knowledge is already with us, either consciously or unconsciously, a rationalist claims we don’t really “learn” things in the traditional usage of the word, but rather that we simply bring to light what we already know.

The innate concept thesis

Rationale: “We have some of the concepts we employ in a particular subject area, S, as part of our rational nature.”[32]

Similar to the Innate Knowledge thesis, the Innate Concept thesis suggests that some concepts are simply part of our rational nature. These concepts are a priori in nature and sense experience is irrelevant to determining the nature of these concepts (though, sense experience can help bring the concepts to our conscious mind).

Some philosophers, such as John Locke (who is considered one of the most influential thinkers of the Enlightenment and an empiricist) argue that the Innate Knowledge thesis and the Innate Concept thesis are the same.[33] Other philosophers, such as Peter Carruthers, argue that the two theses are distinct from one another. As with the other theses covered under the umbrella of rationalism, the more types and greater number of concepts a philosopher claims to be innate, the more controversial and radical their position; “the more a concept seems removed from experience and the mental operations we can perform on experience 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 for being innate than our concept of the latter.[32]

In his book, Meditations on First Philosophy,[34] René Descartes postulates three classifications for our ideas when he says, “Among my ideas, some appear to be innate, some to be adventitious, and others to have been invented by me. My understanding of what a thing is, what truth is, and what thought is, seems to derive simply from my own nature. But my hearing a noise, as I do now, or seeing the sun, or feeling the fire, comes from things which are located outside me, or so I have hitherto judged. Lastly, sirens, hippogriffs and the like are my own invention.”[35]

Adventitious ideas are those concepts that we gain through sense experiences, ideas such as the sensation of heat, because they originate from outside sources; transmitting their own likeness rather than something else and something you simply cannot will away. Ideas invented by us, such as those found in mythology, legends, and fairy tales are created by us from other ideas we possess. Lastly, innate ideas, such as our ideas of perfection, are those ideas we have as a result of mental processes that are beyond what experience can directly or indirectly provide.

Gottfried Wilhelm Leibniz defends the idea of innate concepts by suggesting the mind plays a role in determining the nature of concepts, to explain this, he likens the mind to a block of marble in the New Essays on Human Understanding, “This is why I have taken as an illustration a block of veined marble, rather than a wholly uniform block or blank tablets, that is to say what is called tabula rasa in the language of the philosophers. For if the soul were like those blank tablets, truths would be in us in the same way as the figure of Hercules is in a block of marble, when the marble is completely indifferent whether it receives this or some other figure. But if there were veins in the stone which marked out the figure of Hercules rather than other figures, this stone would be more determined thereto, and Hercules would be as it were in some manner innate in it, although labour would be needed to uncover the veins, and to clear them by polishing, and by cutting away what prevents them from appearing. It is in this way that ideas and truths are innate in us, like natural inclinations and dispositions, natural habits or potentialities, and not like activities, although these potentialities are always accompanied by some activities which correspond to them, though they are often imperceptible.

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