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A consequence of the new ‘mathematicized’ physics of Descartes was that a certain gap opened up between the quantitative description of the world put forward by the scientist and the ‘common sense’ world revealed by the five senses – the world of colours, smells, tastes, sounds and textures. Descartes had pointed out that nothing reaches the brain from the outside world except various ‘local motions’ transmitted via the sense organs; and he concluded that ‘the properties in external objects to which we apply the terms “light”, “colour”, “smell”, “taste”, “sound”, “heat” and “cold”… are so far as we can see simply various dispositions in the shapes, sizes, positions and movements of their parts that make them able to set up various kinds of motions in our nerves, which then produce all the various sensations in our soul’.¹ Taking up this theme, the English philosopher John Locke made a radical distinction between primary and secondary qualities of things. Primary qualities such as shape, he argues in the following extract from the Essay concerning Human Understanding (which appeared at the end of 1689²), are ‘utterly inseparable from the Body in whatsoever state it be’. Descartes, as we have seen, took being extended in three dimensions as the essential characteristic of matter; Locke’s list of the basic or primary qualities of matter comprises ‘solidity, extension, figure [i.e. shape] and mobility’.

Now as far as our ordinary idea of the objects around us is concerned, we normally conceive of them as having many other qualities in addition to those on Locke’s list – the marigold has a striking colour, the pineapple a characteristic taste, the perfume a distinctive aroma, and so on. But such qualities are, for Locke, merely powers which objects have to produce various sensations in us by means of their primary qualities (here Locke again owes much to his illustrious French predecessor: compare the sentence from Descartes quoted in the previous paragraph). The upshot is that when we conceive of an object as having shape, for example, there is, as Locke puts it, a ‘resemblance’ between our idea of the object and how it really is: ‘[the] patterns do really exist in the Bodies themselves’. But when we call an object ‘sweet’ or ‘blue’, ‘there is nothing like our ideas existing in the bodies themselves’.

The real physical world thus turns out to be, for Locke, very different in nature from how we often naïvely suppose it to be. A kind of veil interposes itself between our human sensory awareness of the world on the one hand, and, on the other, the world as it ‘really is’ – the world of corpuscular physics consisting simply of solid, extended, moving particles. Some readers may feel that this distinction between appearance and reality is indeed forced on us by the discoveries of science. But we need to be on our guard. Does science actually show us that a marigold is not ‘really’ coloured? Indeed, on Locke’s own account, if the redness of a rose is a ‘power’ to produce certain sensations in us, does this not imply that the flower does genuinely possess that power? Finally if a metaphysical veil is to be drawn between some of our ideas and how things really are ‘out there’, one might begin to ask whether any of our ideas can support the notion of ‘what really exists in the bodies themselves’ (for more on this, see extract 6, below).

To discover the nature of our ideas the better, and to discourse of them intelligibly, it will be convenient to distinguish them, as they are ideas or perceptions in our minds, and as they are modifications of matter in the bodies that cause such perceptions in us; that so we may not think (as perhaps usually is done) that they are exactly the images and resemblances of something inherent in the subject; most of those [ideas] of sensation being in the mind no more the likeness of something existing without [outside] us, than the names, that stand for them, are the likeness of our ideas, which yet upon hearing, they are apt to excite in us.

Whatsoever the mind perceives in itself, or is the immediate object of perception, thought, or understanding, that I call idea; and the power to produce any idea in our mind, I call quality of the subject wherein that power is. Thus a snowball having the power to produce in us the ideas of white, cold, and round, the powers to produce those ideas in us, as they are in the snowball, I call qualities; and as they are sensations, or perceptions, in our understandings, I call them ideas. Which ideas, if I speak of sometimes as in the things themselves, I would be understood to mean those qualities in the objects which produce them in us.

Qualities thus considered in bodies are, first such as are utterly inseparable from the body, in what estate soever it be; such as in all the alterations and changes it suffers, all the force can be used upon it, it constantly keeps; and such as sense constantly finds in every particle of matter, which has bulk enough to be perceived, and the mind finds inseparable from every particle of matter, though less than to make itself singly be perceived by our senses. E.g., take a grain of wheat, divide it into two parts, each part has still solidity, extension, figure, and mobility; divide it again, and it retains still the same qualities; and so divide it on, till the parts become insensible, they must retain still each of them all those qualities. For division (which is all that a mill, or pestle, or any other body, does upon another, in reducing it to insensible parts) can never take away either solidity, extension, figure, or mobility from any body, but only makes two, or more distinct separate masses of matter, of that which was but one before, all which distinct masses, reckoned as so many distinct bodies, after division make a certain number. These I call original or primary qualities of body, which I think we may observe to produce simple ideas in us, viz. solidity, extension, figure, motion, or rest, and number.

Secondly, such qualities, which in truth are nothing in the objects themselves, but powers to produce various sensations in us by their primary qualities, i.e. by the bulk, figure, texture, and motion of their insensible parts, as colours, sounds, tastes, etc., these I call secondary qualities. To these might be added a third sort which are allowed to be barely powers though they are as much real qualities in the subject, as those which I to comply with the common way of speaking call qualities, but for distinction secondary qualities. For the power in fire to produce a new colour, or consistency in wax or clay by its primary qualities, is as much a quality in fire, as the power it has to produce in me a new idea or sensation of warmth or burning, which I felt not before, by the same primary qualities, viz. the bulk, texture, and motion of its insensible parts.

The next thing to be considered, is how bodies produce ideas in us, and that is manifestly by impulse, the only way which we can conceive bodies operate in.

If then external objects be not united to our minds, when they produce ideas in it; and yet we perceive these original qualities in such of them as singly fall under our senses, it is evident, that some motion must be thence continued by our nerves, or animal spirits,³ by some parts of our bodies, to the brains or the seat of sensation, there to produce in our minds the particular ideas we have of them. And since the extension, figure, number, and motion of bodies of an observable bigness, may be perceived at a distance by the sight, it is evident some singly imperceptible bodies must come from them to the eyes, and thereby convey to the brain some motion, which produces these ideas, which we have of them in us.

After the same manner that the ideas of these original qualities are produced in us, we may conceive that the ideas of secondary qualities are also produced, viz. by the operation of insensible particles on our senses. For it [is] manifest that there are bodies, and good store of bodies, each whereof is so small that we cannot, by any of our senses, discover either their bulk, figure, or motion, as is evident in the particles of the air and water, and others extremely smaller than those – perhaps as much smaller than the particles of air, or water, as the particles of air or water are smaller than peas or hailstones. Let us suppose at present that the different motions and figures, bulk, and number of such particles, affecting the several organs of our senses, produce in us those different sensations which we have from the colours and smells of bodies; e.g. that a violet, by the impulse of such insensible particles of matter of peculiar figures and bulks, and in different degrees and modifications of their motions, causes the ideas of the blue colour, and sweet scent of that flower to be produced in our minds – it being no more impossible to conceive, that God should annex such ideas to such motions, with which they have no similitude, than that he should annex the idea of pain to the motion of a piece of steel dividing our flesh, with which that idea has no resemblance.

What I have said concerning colours and smells may be understood also of tastes and sounds, and other [similar] sensible qualities; which, whatever reality we, by mistake, attribute to them, are in truth nothing in the objects themselves but powers to produce various sensations in us, and depend on those primary qualities, viz. bulk, figure, texture, and motion of parts; as I have said.

From whence I think it is easy to draw this observation, that the ideas of primary qualities of bodies are resemblances of them, and their patterns do really exist in the bodies themselves; but the ideas produced in us by these secondary qualities, have no resemblance of them at all. There is nothing like our ideas existing in the bodies themselves. They are, in the bodies we denominate from them, only a power to produce those sensations in us. And what is sweet, blue, or warm in idea is but the certain bulk, figure, and motion of the insensible parts in the bodies themselves, which we call so.

Flame is denominated hot and light; snow white and cold and manna white and sweet, from the ideas they produce in us. Which qualities are commonly thought to be the same in those bodies that those ideas are in us, the one the perfect resemblance of the other, as they are in a mirror; and it would by most men be judged very extravagant if one should say otherwise. And yet he that will consider that the same fire, that at one distance produces in us the sensation of warmth, does, at a nearer approach, produce in us the far different sensation of pain, ought to bethink himself what reason he has to say that his idea of warmth, which was produced in him by the fire, is actually in the fire; and his idea of pain, which the same fire produced in him the same way, is not in the fire. Why is whiteness and coldness in snow, and pain not, when it produces the one and the other idea in us; and can do neither but by the bulk, figure, number, and motion of its solid parts?

The particular bulk, number, figure, and motion of the parts of fire, or snow, are really in them, whether anyone’s senses perceive them or no; and therefore they may be called real qualities, because they really exist in those bodies. But light, heat, whiteness, or coldness, are no more really in them than sickness or pain is in manna. Take away the sensation of them; let not the eyes see light, or colours, nor the ears hear sounds; let the palate not taste, nor the nose smell, and all colours, tastes, odours, and sounds, as they are such particular ideas, vanish and cease, and are reduced to their causes, i.e. bulk, figure, and motion of parts.

A piece of manna⁴ of a sensible bulk is able to produce in us the idea of a round or square figure; and, by being removed from one place to another, the idea of motion. This idea of motion represents it as it really is in the manna moving. A circle or square are the same, whether in idea or existence – in the mind, or in the manna. And this, [that] both motion and figure are really in the manna, whether we take notice of them or no, this everybody is ready to agree to. Besides, manna, by the bulk, figure, texture, and motion of its parts, has a power to produce the sensations of sickness, and sometimes of acute pains, or gripings in us. That these ideas of sickness and pain are not in the manna, but effects of its operations on us, and are nowhere when we feel them not – this also every one readily agrees to. And yet men are hardly to be brought to think that sweetness and whiteness are not really in manna; which are but the effects of the operations of manna, by the motion, size, and figure of its particles on the eyes and palate; as the pain and sickness caused by manna are confessedly nothing but the effects of its operations on the stomach and guts, by the size, motion, and figure of its insensible parts (for by nothing else can a body operate, as has been proved). As if it could not operate on the eyes and palate, and thereby produce in the mind particular distinct ideas, which in itself it has not, as well as we allow it can operate on the guts and stomach, and thereby produce distinct ideas, which in itself it has not. These ideas being all effects of the operations of manna on several parts of our bodies, by the size, figure, number, and motion of its parts, why those produced by the eyes and palate should rather be thought to be really in the manna, than those produced by the stomach and guts; or why the pain and sickness, ideas that are the effects of manna, should be thought to be nowhere, when they are not felt; and yet the sweetness and whiteness, effects of the same manna on other parts of the body, by ways equally as unknown, should be thought to exist in the manna, when they are not seen nor tasted, would need some reason to explain.

Let us consider the red and white colours in porphyry. Hinder light but from striking on it, and its colours vanish; it no longer produces any such ideas in us. Upon the return of light, it produces these appearances on us again. Can any one think any real alterations are made in the porphyry by the presence or absence of light; and that those ideas of whiteness and redness, are really in porphyry in the light, when it is plain it has no colour in the dark? It has, indeed, such a configuration of particles, both night and day, as are apt by the rays of light rebounding from some parts of that hard stone, to produce in us the idea of redness, and from others the idea of whiteness. But whiteness or redness are not in it at any time, but such a texture that has the power to produce such a sensation in us.

Pound an almond, and the clear white colour will be altered into a dirty one, and the sweet taste into an oily one. What real alteration can the beating of the pestle make in any body, but an alteration of the texture of it?

Ideas being thus distinguished and understood, we may be able to give an account, how the same water, at the same time, may produce the idea of cold by one hand, and of heat by the other: Whereas it is impossible, that the same water, if those ideas were really in it, should at the same time be both hot and cold. For if we imagine warmth, as it is 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 may understand how it is possible that the same water may at the same time produce the sensation of heat in one hand, and cold in the other; which yet figure never does, that never producing the idea of a square by one hand, which has produced the idea of a globe by another. But if the sensation of heat and cold be nothing but the increase or diminution of the motion of the minute parts of our bodies, caused by the corpuscles of any other body, it is easy to be understood: that if that motion be greater in one hand than in the other, if a body be applied to the two hands, which has in its minute particles a greater motion than in those of one of the hands, and a less than in those of the other, it will increase the motion of the one hand and lessen it in the other, and so cause the different sensations of heat and cold that depend thereon.

I have, in what just goes before, been engaged in physical enquiries a little farther than, perhaps, I intended. But it being necessary to make the nature of sensation a little understood, and to make the differences between the qualities in bodies and the ideas produced by them in the mind to be distinctly conceived, without which it were impossible to discourse intelligibly of them; I hope, I shall be pardoned this little excursion into natural philosophy, it being necessary in our present enquiry, to distinguish the primary, and real qualities of bodies, which are always in them (viz. solidity, extension, figure, number, and motion, or rest), and are sometimes perceived by us (viz. when the bodies they are in are big enough singly to be discerned), from those secondary and imputed qualities, which are but the powers of several combinations of those primary ones, when they operate without being distinctly discerned; whereby we also may come to know what ideas are, and what are not, resemblances of something really existing in the bodies we denominate from them.