The study of knowledge
Epistemology, a modern term for the theory of knowledge, is the branch of philosophy that studies the possibility, nature, and scope of human knowledge. The first extant attempt to account for knowledge is recorded in the works of Plato. His basic assumption is that knowledge must be of something that is constant and invariable. This at once rules out the world of ordinary experience, since that concerns things that are always in flux, as Heraclitus had taught. Therefore what we learn through sense experience cannot be knowledge.
Since different people experience things differently, Plato argues, we should be driven into the position of the Sophists, that everybody's opinion is valid for him, and knowledge would become subjective and quite arbitrary.
The opposite view, that knowledge excludes all change and therefore all difference, leads to the view of Parmenides, that reality is a uniform, static, spherical whole. Plato's solution is that knowledge is of a set of abstract entities that are 'Ideas' (which is Greek for pictures) of things we ordinarily experience. These Ideas or Forms are eternal, unchanging, and resident in a realm of their own; and they alone are real.
Knowledge is of Ideas
As regards ordinary things, they become amenable to knowledge by somehow partaking in the corresponding Ideas, although this relation of participation turns out in the end to be untenable, as Plato himself came to see. By contrast, sense experience gives rise only to opinion or belief, which falls short of knowledge. Moreover, Plato showed that knowledge cannot be defined in terms of belief. Whereas for Plato the Forms were thus transcendent (existing beyond the world of things), for Aristotle they were immanent (existing in the various particular things of our world). Forms cannot in fact be separated from real things, but only considered abstractly apart in thought: the mind does not create them, they really exist, but not in separation. The Forms are what were later called universals. In medieval philosophy, the nature of universals was much debated. Some supported the view that they were real, whether before things as for Plato, or in things as for Aristotle; while others developed the view that they were mere names, products of the mind. Amongst the former, called Realists, was St Thomas Aquinas, the greatest of the medieval followers of Aristotle; while the latter, called Nominalists, broke away from the metaphysics of Plato and Aristotle and, especially with William of Ockham, point in the direction of the British empiricists.
It was not until the advent of the scientific revolution at the turn of the 17th century that philosophers began to take a renewed interest in epistemology. The Realist position issued in the rationalist theory put forward by Rene Descartes and variously developed afterwards. On this view, nothing was to count as acceptable unless it complied with the principles of reason. Anything in the least subject to doubt must be rejected. In this way we are gradually forced to abandon all our ordinary beliefs until we finally reach a proposition that defies doubt; it can then serve as a starting point of knowledge, and will provide a criterion of certainty by which to judge any other claims to knowledge. This indubitable starting point Descartes finds in the statement 'I think, therefore I am'. Sense experience, so often deceptive, is ruled out as the source of knowledge, whereas the method of doubt leads us to this rational principle, clearly and distinctly perceived. Descartes also recommends that we divide complex problems into simple parts and proceed from the latter. This procedure, which really stems from an examination of mathematical questions, thus seeks to assimilate all knowledge to mathematics, a view that ultimately goes back to Pythagoras (the Greek word 'mathematics' does indeed mean literally 'the art of learning'). As to error, this occurs if we depart from the method and make judgments without proper knowledge.
Since rational principles do not come from the senses, they must be innate. This runs directly counter to the empiricist notion, developed in the later 17th century, that all knowledge comes ultimately from sense experience. Locke thought that the mind is to begin with like a blank tablet on which the experience of the senses engraves impressions. As to the operations of the mind itself, these are grasped through introspective experience. Thus we have the twin sources of ideas of sensation and ideas of reflection. On this view it becomes difficult to see how one can speak of an external world that produces the sensations.
This point was taken up by Berkeley, who argued that there were only perceptions, and that for something to exist is the same thing as to be perceived. As for the mind, it was constituted by perceiving. However, on this basis we cannot give an account of that mind. Hume took the final step and denied that there was a perceiving mind or self: there are only bundles of perceptions, either vivid and direct or faint and indirect.
These he calls impressions and ideas respectively. Why particular bundles of perception should belong together once more remains inexplicable. With Kant the problem of knowledge is subjected to a new and critical approach. He admits that all knowledge arises through experience; but it does not therefore derive from experience. Indeed it is the mind itself that supplies the general framework that makes experience possible: it is not the world impressing itself on the mind, but the mind impressing itself on the world that produces knowledge.
In contemporary philosophy these various theories have been further developed. In the end a certain disenchantment with all these inconclusive theories has led to contemporary attempts to seek a solution within language itself, by analyzing the meaning of claims to knowledge.
Do you believe all knowledge is gained from experience or acquired through pure reason alone?