John Locke belongs to the epistemological school of thought called Empiricism. Empiricism is a reaction to Rationalism which holds that reason, (in some sense), is the only road to genuine knowledge. The three notable rationalists are Rene Descartes, Wilhelm Leibniz and Baruch Spinoza. These rationalist philosophers have tried to find a completely certain foundation for our knowledge in terms of certain procedures of human reasoning.
On the contrary, Empiricism is a theory of knowledge which holds that experience, rather than reason is the source of knowledge. The modern empiricists were John Locke, George Berkeley and David Hume and they held that no innate knowledge exists and that whatever knowledge man possesses is acquired through experience. In other words, they asserted that all genuine knowledge is derived from sense perception and denied that reason alone without the sense can acquire any genuine knowledge.
The beginning of Locke‘s epistemology is the rejection of the rationalist doctrine that men have innate knowledge of some truth either moral or speculative, which supplies the foundations of knowledge. Against this background, Locke argued that experience is the source and basis of knowledge. He argued against Plato, Descartes and the Scholastics, that there are no innate ideas or principles. According to him, experience gives rise to various kinds of idea. On the basis of sense experience, he tried to construct an account of knowledge. For Locke knowledge is ideas, but not Plato‘s ideas or Forms, but ideas that are generated from experience.
Locke held that knowledge begins with ideas which are generated by experience. For him, idea is that object about which the understanding is concerned with while thinking. Our ideas are derived from two sources (a) sensation (b) perception of the operation of our mind, which may be called ‗internal sense‘ (reflection). Since we can only think by means of ideas, and since all ideas come from experience, it is evident that none of our knowledge can antedate experience.
The mind originally empty and blank receives, simple ideas of two kinds – those of sensation and those of reflection. Simple ideas of sensation are furnished by ‗external objects‘ or bodies, which produce ideas in us by mechanical action upon our organism, by impulse, the only way which we can conceive bodies to operate in. We have five primary senses through which external world is known to us. They are the senses of sight, hearing, smelling, feeling or touching and taste. Each of these media communicates the external object in terms of idea of colour, sound, odour, hardness and sweetness. The second of simple ideas is reflection of the mind upon its own operation as it is employed about the ideas it has got.
Locke also distinguished our ideas into simple and complex ideas. Simple ideas, such as yellow, heat, sweet, hard, constitute the chief sources of the raw materials out of which our knowledge is made. These ideas are directly caused by things but are passively received by the mind through the senses. These ideas are not exactly the things rather they are copies or representations of things in our minds when they impress themselves (things) on our minds through the senses. The theory that ideas are not the same with physical objects but copies representations or resemblance of the physical objects is known as Representative theory. Complex ideas on the other hand are combinations of simple ideas. Here the emphasis is upon the activity of the mind, which takes three forms – the mind joins ideas, bring ideas together but holds them separate, and abstract. Hence, the mind joins the ideas of whiteness, hardness, squareness and sweetness to form the complex idea of a cube of sugar. The mind also brings ideas together but holds them separate for the purpose of thinking of relationship.
In a conclusion, Locke thinks of knowledge of the external world as sensitive knowledge of real existence. That is, it is knowledge that some object exists distinct from our mind and affects our mind by producing certain ideas in it. This knowledge is achieved through sensory experience. It is neither the result of reflecting on ideas already in our mind nor of deductively reasoning from some premises.