Traditionally, philosophers have thought that if someone knows X, that means that he or she (1) believes that X is true, (2) X is, in fact, true, and (3) the person who claims to know X can give a justification or rationale for thinking that X is true. Such a justification can be given by appealing to intuition (immediate, personal certainty that X is true), reasoning (proving that X is true based on shared strategies of argumentation), or sense experience (public, repeatable, verifiable demonstration or experiment showing that X is true).
Despite the fact that intuition is a common phenomenon, philosophers have often been hesitant to identify it as a form of knowledge--primarily because there seems to be little way to determine whether it does, in fact, provide knowledge as opposed simply to lucky guesses. So most philosophers focus, instead, on reason and sense experience as the bases of knowledge. These two latter ways of approaching the question of knowledge are identified as rationalism and empiricism.
A rationalist epistemology claims that knowledge (as opposed to opinion) is possible only if it is based on self-evident and absolutely certain principles. Such principles are not learned through experience; instead, they are implicit in the very notion of reasoning (in Latin: ratio) itself. Sense experience cannot provide the certainty needed to guarantee that what we claim to know is true. So, like mathematicians, we have to rely on reason itself as the basis for determining whether our opinions are justified true beliefs (that is, knowledge).
Plato is an example of a rationalist. He says that sense experience fails to provide us with any guarantee that what we experience is, in fact, true. The information we get by relying on sense experience is constantly changing and is often unreliable. It can be corrected and evaluated for dependability only be appealing to principles that themselves do not change. These unchanging principles (or "Forms") are the bases of what it means to think or reason in the first place. So if someone can show that an opinion or belief he or she has is based on these undoubtable principles of thought, he or she has a firm foundation for the opinion. That foundation is what allows us to think of a belief as more than simply opinion; it is what allows us to identify the belief as justified and true, and that is what is meant by knowledge.
Knowledge for the rationalist is thus what can be deduced from principles that cannot be otherwise; they are undoubtable ("indubitable"). Examples of such principles include: "Bachelors are unmarried males," "A thing cannot be and not be at the same time in the same way," "Triangles have three sides," and "A whole is always greater than any one of its parts." These statements are known with certainty to be true because the very meaning of the terms involved (e.g., bachelors, triangles, things, wholes) requires that we think of them in certain ways (without relying on sense experience). We thus know about some things prior to any sense experience we have or could have. Such knowledge is called a priori. Any knowledge that relies on (that is, comes after or is posterior to) sense experience is called a posteriori.
Rene' Descartes (1596-1650) is another example of a rationalist. Instead of beginning philosophical inquiry (like the Milesians) with the study of the nature of reality, he suggests that we ask what it would mean to know about reality. To believe that reality is fundamentally water or the Indeterminate or whatever seems pointless, he claims, unless we know first whether our belief itself is justified. To determine whether our beliefs are justified, we have to be able to trace them back to a statement, belief, or proposition that cannot be doubted. Such a proposition could provide the firm foundation on which all subsequent beliefs could be grounded; it would guarantee that all subsequent claims based on it would be true.
In order to identify an ultimate principle of truth on which all other knowledge can be based, Descartes develops a method that suspends our confidence in what we have been taught, what our senses tell us, what we "think" is obvious--in short, in regard to everything we know. In order to determine whether there is anything we can know with certainty, he says that we first have to doubt everything we know. Such a radical doubt might not seem reasonable, and Descartes certainly does not mean that we really should doubt everything. What he suggests is that, in order to see if there is some belief that cannot be doubted, we should temporarily pretend that everything we know is questionable.
Since sense experience is sometimes deceiving, it is obvious to Descartes that a posteriori claims (e.g., that this milk tastes sour or that suit is dark blue) cannot be the basis for claims of knowledge. We do not know that what we experience through our senses is true; at least, we are not certain of it. So the best thing to do is to doubt our senses. Likewise, we cannot be sure that we really have bodies or that our experience of the world in general can be trusted; after all, we might be dreaming the whole thing. Next, we cannot even be sure that mathematical propositions such as 2+3=5 or that triangles always have three sides are true because some evil power might be deceiving us to think such things, when it is possible that even propositions that seem evident to us as true might themselves be really false. But even if an evil genie deceives us about all other beliefs, there is one belief that we cannot be mistaken about, and that is that we are thinking. Even to doubt this is to affirm it. Thinking proves that we exist (at least as minds or thinking things, regardless of whether we have bodies). The body is not an essential part of the self because we can doubt its existence in a way that we cannot doubt the existence of the mind.
So Descartes concludes that I know one thing clearly and distinctly, namely, that I exist because I think: "Cogito ergo sum," I think, therefore I exist. From this starting point I can begin to note other truths that I know clearly and distinctly, such as the principle of identity (A is A) and the notion that things in the world are "substances." Since identity and substance are ideas that are not based on sensation, they must be innate (that is, they must be implicit in the very act of thinking itself). Even sensible things (e.g., a block of wax) are knowable not based on sense experience but intellectually, insofar as we know them to be the same things even though their sensible appearances might change dramatically.
In order to be certain that we are not deceived when we claim to know something, Descartes must dispose of the evil genie. This is done by proving that an all-good, all-powerful God would not permit us to be deceived. If there is such a God, we can have knowledge. Since the senses cannot be trusted to provide a proof that God exists, only a proof based on the principle of the cogito ("I think, therefore I am") will work. That proof can be summarized in the following way:
I know I exist; but the "I" who exists is obviously imperfect; otherwise I would not have doubts about what I know in the first place. To know that I, an imperfect thing, exist means that I already know that a perfect thing must exist in terms of which my own existence is meaningful. I know what it means to be imperfect only if I already know what perfection is. But I do not know perfection in virtue of my self; therefore there must be a perfect substance (God) who exists in terms of which my own imperfect existence is intelligible. No perfect (all-good, all-powerful) being would deceive us into thinking that we know something with certainty when, in fact, we are mistaken about it. So if there is a God, then no evil genie could exist who tricks us regarding clear and distinct knowledge (such as mathematical reasoning).
We have a "great inclination" to believe that there are physical objects that are external to the mind. But since only those objects known in terms of mathematical properties--not those imagined by use of the senses--can be known clearly and distinctly, the only knowledge we can have of such objects is in terms of mathematical, quantifiable physics. The only real knowledge we can have, then, is of things understood as functions of laws of physics. The objects we see are not the objects we know, because what we know is intelligible only in terms of the clarity and precision of the formulae of physics. Information provided by the senses cannot therefore be the basis of knowledge.
Certitude is thus grounded in the knowledge of the self, which is itself intelligible only if there is a God who guarantees that we are not deceived about what we know of the world clearly and distinctly (i.e., mathematically). By appeal to reason alone, we are able to know: this is the main message of rationalism.
Objections to Rationalism: