Neither would it carry any Imputation of Falsehood to our simple Ideas, if by the different Structure of our Organs, it were so ordered, That the same Object should produce in several Men’s Minds different Ideas at the same time; e.g. if the Idea, that a Violet produced in one Man’s Mind by his Eyes, were the same that a Marigold produces in another Man’s, and vice versa. For since this could never be known: because one Man’s Mind could not pass into another Man’s Body, to perceive, what Appearances were produced by those Organs; neither the Ideas hereby, nor the Names, would be at all confounded, or any Falsehood be in either.

Let us then suppose the mind to have no ideas in it, to be like white paper with nothing written on it. How then does it come to be written on? From where does it get that vast store which the busy and boundless imagination of man has painted on it—all the materials of reason and knowledge? To this I answer, in one word, from experience. Our understandings derive all the materials of thinking from observations that we make of •external objects that can be perceived through the senses, and of •the internal operations of our minds, which we perceive by looking in at ourselves. These two are the fountains of knowledge, from which arise all the ideas we have or can naturally have.

Qualities thus considered in bodies are of two kinds. First, there are those that are utterly inseparable from the body, whatever state it is in. Qualities of this kind are the ones that a body doesn’t lose, however much it alters, whatever force is used on it, however finely it is divided. Take a grain of wheat, divide it into two parts, each part has still solidity, extension, shape, and mobility; divide it again, and it still retains those qualities; go on dividing it until the parts become imperceptible, each part must still retain all those qualities. . . . I call them original or primary qualities of body, which I think we may observe to produce simple ideas in us, viz. solidity, extension, shape, motion or rest, and number.

Secondly, there are qualities that are, in the objects themselves, really nothing but powers to produce various sensations in us by their primary qualities, i.e. by the size, shape, texture, and motion of their imperceptible parts. Examples of these are colours, sounds, tastes, and so on. I call these secondary qualities. To these we can add a third sort, an example of which is the power of fire to change the colour or consistency of wax and clay. This would ordinarily be said to be only a power in ·rather than a quality of · the object; but it is just as much a real quality as the powers that I have called ‘secondary qualities’. (I call them ‘qualities’ so as to comply with the common way of speaking, and add ‘secondary’ to mark them off from the rest.) The primary qualities of fire—that is, the size, texture, and motion of its minute parts—give it a power to affect wax and clay etc.; and those same primary qualities give it a power to produce in me a sensation of warmth or burning; if the latter is a quality in the fire, why not the former also? 

We are now in a position to explain how it can happen that the same water, at the same time, produces the idea of cold by one hand and of heat by the other; whereas the same water couldn’t possibly be at once hot and cold if those ideas were really in it. If we imagine warmth in our hands to be nothing but a certain sort and degree of motion in the minute particles of our nerves or animal spirits, we can understand how it is possible for the same water at the same time to produce the sensations of heat in one hand and of cold in the other (which shape never does; something never feels square to one hand and spherical to the other). If the sensation of heat and cold is nothing but the increase or lessening of the motion of the minute parts of our bodies, caused by the corpuscles of some other body, we can easily understand that if motion is greater in one hand than in the other, and the two hands come into contact with a body that is intermediate between them in temperature, the particles in one hand will be slowed down while those of the other will speed up, thus causing different sensations.

To find what personal identity consists in, we must consider what ‘person’ stands for. I think it is a thinking intelligent being, that has reason and reflection, and can consider itself as itself, the same thinking thing at different times and places. What enables it to think of itself is its consciousness, which is inseparable from thinking and (it seems to me) essential to it. It is impossible for anyone to perceive, without perceiving that he perceives. When we see, hear, smell, taste, feel, meditate, or will anything, we know that we do so. It is always like that with our present sensations and perceptions. And it is through this that everyone is to himself that which he calls ‘self’, not raising the question of whether the same self is continued in the same substance. Consciousness always accompanies thinking, and makes everyone to be what he calls ‘self’ and thereby distinguishes himself from all other thinking things; in this alone consists personal identity, i.e. the sameness of a rational being; and as far as this consciousness can be extended backwards to any past action or thought, so far reaches the identity of that person; it is the same self now that it was then; and this present self that now reflects on it is the one by which that action was performed.