John Locke, as an empiricist, refutes the rationalistic doctrine of innate ideas. There are no inborn truths for Locke. All knowledge is empirical, received through the senses. The mind has no private truths. It is originally a tabula rasa, a blank tablet, on which external things make their impression through the senses. Even our inward ideas are products of outward sensations. The mind cannot have its own ideas independent of sense-perception. We know nothing that is not perceived through the senses or reflected by the mind on the basis of sense-perception. Sensuous and reflective experience is therefore the ground of all our knowledge. Sensation and reflection constitute the whole of our experience. The mind formulates ideas and reflects on the basis of sense-perception. Simple ideas received by means of sense can be converted into complex ideas by the mind; but the mind does not create new ideas, nor destroy them.

Locke distinguishes between ideas produced by mere sensation, which may not correspond to the actual properties of things outside, and those which really correspond to them. The qualities of things which create ideas through sensations and which do not correspond to their real properties are called secondary qualities, while those qualities in things which produce sensations and ideas corresponding to their inherent properties are called primary qualities. Solidity, extension etc. are considered to be the real properties of things and so they are primary qualities, while colours, sounds etc. are not qualities inherent in things, and so they are secondary qualities. The primary qualities are really present in things, while the secondary ones are not. Our knowledge is confined to the perception of the secondary and the primary qualities, received through sensations, external and internal, though the mind can convert our simple ideas of these sensations into complex ones. Our ideas of things or substances are derived by sensation and reflection; the substances are merely assumed as existent on account of the sensation of the qualities and the formation of the ideas. Substance, mode and relation are just complex ideas of sensations and cannot pretend to be anything more. We have only a representation in our minds of the real things outside; we do not perceive them directly. What we know are only the secondary and primary qualities, not the substances in which they inhere.

The world outside is independent of the mind. It is the presence of the real objects that causes in us real sensations. The world consists of substances, in which qualities and actions inhere. According to Locke, there are two kinds of substances: bodies and souls. We perceive bodies and have a clear and immediate idea of our soul. We know bodies through sensation and the soul by reflection. Thought is an activity which inheres in the soul. Bodies are material and souls immaterial. From the perception of physical qualities their basic substance is assumed, for qualities cannot simply hang in the air, they must have a substratum. Similarly from the observation of mental operations, a notion of their basis, a spiritual soul, is formed.

Locke admits that there is interaction between body and mind, both of which are real beings. All our ideas are the results of the action of bodies on our minds. The soul experiences changes on account of its being acted on by bodies outside. Locke does not think that our perception of the external world is clearer than our notion of the reality or existence of the soul, or that we are surer of the nature of bodies than that of souls. He would rather say that our idea of the soul and its action is clearer and more distinct than that we have regarding material bodies. Our knowledge of bodies outside is not certain knowledge; the secondary qualities which we perceive do no represent the reality of things. The secondary qualities are produced not by the things as such, but by the primary qualities which inhere in things and which really belong to things. The primary qualities really represent things.

But Locke tells us that bodies affect not minds or consciousness, but only bodies, and physical motion can affect only physical motion. How, then, can Locke justify his theory of representationism, which holds that we receive mental images of physical substances that exist outside in reality? This is a difficulty which Locke does not seek to solve. He merely adds that this is possible on account of God's arranging the properties of bodies and of motion in such a way that they can act thus. He, however, becomes bold when he says that we cannot even understand how bodies act on bodies, or how motion produces motion. When we are content to be ignorant of this mystery, why not hold the same attitude towards the action of bodies on senses and minds, seems to be Locke's rejoinder to our objection to his theory of knowledge. We end in mystery. He is satisfied with telling us that we have sensations in this way, and there ends the matter. He is not concerned with the question how they are caused.

Locke is sometimes very candid in doubting whether it is minds or souls alone that think or whether matter, too, can think. When we do not know the essential nature of things, how can we say that minds alone think and not matter? Perhaps what we call soul is only matter, and perhaps matter can be conscious. Locke's misgivings in regard to this problem lose much of their value when we become alive to the fact that what is important is not whether the source of consciousness is matter or mind but that consciousness is the essential characteristic of experience. When we attribute consciousness to matter, what we actually do is to deny the materiality of matter, and to make it a conscious entity; in other words, what we apparently call matter becomes soul in reality. Anyway, the fact remains that the essential nature of the Self or the experiencer is consciousness, name it matter or soul.

In spite of these misgivings that he has, Locke appeals to commonsense and admits that there are two substances: material and mental. Material bodies, according to him, are constituted of minute corpuscles or atoms (or perhaps molecules) which are endowed with the primary qualities. These form the essential active elements of which matter is the embodiment. These again are the bases of the secondary qualities. But Locke says that we cannot know these corpuscles, what their properties are, how they are united, how they act or move. One is tempted to add that Locke could have as well said that we do not know what matter consists of. For his corpuscular theory does not in any way increase the fund of our knowledge. It only states something together with a note that we do not know what is thus stated. That the constitution of matter empirically presents itself as a conglomeration of minute particles,—call these corpuscles, atoms or molecules,—the Vedanta has no objection to admit. For it matter is governed by the laws of space-time and mechanical motion, as long as our perception or observation of matter is limited to the laws to which space, time and causation are subject. Only it would add that this is not all that we have to say about matter. Matter has a higher nature and purpose, which the senses cannot comprehend, and which points to the realisation of a perfection that transcends human nature and its laws. Physical and chemical laws are not denied; only we are advised not to forget that these laws are valid only in the phenomenal world of sense and understanding and that they cannot pretend to explain the final nature of things. Reality is not confined to what we experience empirically.

Locke, like Descartes, admits a third substance, viz., God. He tries to prove the existence of God not from innate ideas, as Descartes has done,—but from sense-experience. According to Locke, we form an idea of God by enlarging or carrying to infinity the laws and objects of our sensations and reflections. Existence, extension, knowledge, power etc. are what we experience, and their infinitude is our idea of God. We do not know God's essence or reality. To the Vedanta, the existence of God is known intuitively, not through sensations. It is not possible for us to form an idea of unity by accumulating the materials supplied to us by the senses. A collection of particulars may give us a vast universe of plurality, but our conception of God points to the reality of something which is not only an undivided wholeness but Consciousness in essence. Consciousness does not become an object of the senses, for what we know through the senses are material bodies. We have an immediate intuitive perception of the existence of Consciousness, which is not deduced from some other premises. Consciousness itself is the fundamental premise from which all other facts are experienced or logically deduced. No doubt, this intuitive perception of the existence of an Infinite Being or Consciousness is not very clear and remains indistinct and hazy in ordinary individuals, and so this admission of the fact of the existence of an Infinite Consciousness is clothed in empirical attributes, such as unlimited extension in space, endless existence in time, limitless knowledge and power, and so on. The Vedanta further says that the essential characteristics of God, as we conceive of Him, are the opposites of the experiences we have in ordinary life. We observe that the world is changing and so we conceive of God as its changeless and eternal substratum. We perceive that objects of our knowledge are inert in nature and so we endow God with supreme intelligence. We experience limitation and pain here; so we conceive of God as absolute freedom and bliss. But it does not mean that God is merely an embodiment of these negative attributes which appear to be the counter-correlatives of relative experience. God, to the Vedanta, is above our conception of existence, knowledge, power and bliss. He is absolutely transcendent and His positive nature cannot be known by us except in direct realisation. Locke also gives us the usual cosmological and teleological proofs for the existence of God, stating that man, who, he knows, is a real being, must have a cause, and the eternal cause of all real beings must be a perfect being that exists and is real. In fine, it is to be noted that Locke's position that objects exist, but they cannot be known; that the soul exists, but it cannot be known; and that God exists, but He cannot be known, leads to great difficulties which he did not foresee, and naturally gives way to the conclusion that we know nothing at all except only sensations and ideas. He paves the way to the mentalism of Berkeley and the scepticism of Hume.