Chapter III. The Nature of Matter

by Bertrand Russell

　　In the preceding chapter we agreed, though without being able to finddemonstrative reasons, that it is rational to believe that our sense-data—forexample, those which we regard as associated with my table—arereally signs of the existence of something independent of us and ourperceptions. That is to say, over and above the sensations of colour,hardness, noise, and so on, which make up the appearance of the table tome, I assume that there is something else, of which these things areappearances. The colour ceases to exist if I shut my eyes, the sensationof hardness ceases to exist if I remove my arm from contact with thetable, the sound ceases to exist if I cease to rap the table with myknuckles. But I do not believe that when all these things cease the tableceases. On the contrary, I believe that it is because the table existscontinuously that all these sense-data will reappear when I open my eyes,replace my arm, and begin again to rap with my knuckles. The question wehave to consider in this chapter is: What is the nature of this realtable, which persists independently of my perception of it?

　　To this question physical science gives an answer, somewhat incomplete itis true, and in part still very hypothetical, but yet deserving of respectso far as it goes. Physical science, more or less unconsciously, hasdrifted into the view that all natural phenomena ought to be reduced tomotions. Light and heat and sound are all due to wave-motions, whichtravel from the body emitting them to the person who sees light or feelsheat or hears sound. That which has the wave-motion is either aether or'gross matter', but in either case is what the philosopher would callmatter. The only properties which science assigns to it are position inspace, and the power of motion according to the laws of motion. Sciencedoes not deny that it may have other properties; but if so, suchother properties are not useful to the man of science, and in no wayassist him in explaining the phenomena.

　　It is sometimes said that 'light is a form of wave-motion', butthis is misleading, for the light which we immediately see, which we knowdirectly by means of our senses, is not a form of wave-motion, butsomething quite different—something which we all know if we are notblind, though we cannot describe it so as to convey our knowledge to a manwho is blind. A wave-motion, on the contrary, could quite well bedescribed to a blind man, since he can acquire a knowledge of space by thesense of touch; and he can experience a wave-motion by a sea voyage almostas well as we can. But this, which a blind man can understand, is not whatwe mean by light: we mean by light just that which a blindman can never understand, and which we can never describe to him.

　　Now this something, which all of us who are not blind know, is not,according to science, really to be found in the outer world: it issomething caused by the action of certain waves upon the eyes and nervesand brain of the person who sees the light. When it is said that light iswaves, what is really meant is that waves are the physical cause of oursensations of light. But light itself, the thing which seeing peopleexperience and blind people do not, is not supposed by science to form anypart of the world that is independent of us and our senses. And verysimilar remarks would apply to other kinds of sensations.

　　It is not only colours and sounds and so on that are absent from thescientific world of matter, but also space as we get it throughsight or touch. It is essential to science that its matter should be in aspace, but the space in which it is cannot be exactly the space we see orfeel. To begin with, space as we see it is not the same as space as we getit by the sense of touch; it is only by experience in infancy that welearn how to touch things we see, or how to get a sight of things which wefeel touching us. But the space of science is neutral as between touch andsight; thus it cannot be either the space of touch or the space of sight.

　　Again, different people see the same object as of different shapes,according to their point of view. A circular coin, for example, though weshould always judge it to be circular, will look oval unlesswe are straight in front of it. When we judge that it is circular,we are judging that it has a real shape which is not its apparent shape,but belongs to it intrinsically apart from its appearance. But this realshape, which is what concerns science, must be in a real space, not thesame as anybody's apparent space. The real space is public, theapparent space is private to the percipient. In different people's privatespaces the same object seems to have different shapes; thus the realspace, in which it has its real shape, must be different from the privatespaces. The space of science, therefore, though connected with thespaces we see and feel, is not identical with them, and the manner of itsconnexion requires investigation.

　　We agreed provisionally that physical objects cannot be quite like oursense-data, but may be regarded as causing our sensations. Thesephysical objects are in the space of science, which we may call 'physical'space. It is important to notice that, if our sensations are to be causedby physical objects, there must be a physical space containing theseobjects and our sense-organs and nerves and brain. We get a sensation oftouch from an object when we are in contact with it; that is to say, whensome part of our body occupies a place in physical space quite close tothe space occupied by the object. We see an object (roughly speaking) whenno opaque body is between the object and our eyes in physical space.Similarly, we only hear or smell or taste an object when we aresufficiently near to it, or when it touches the tongue, or has somesuitable position in physical space relatively to our body. We cannotbegin to state what different sensations we shall derive from a givenobject under different circumstances unless we regard the object and ourbody as both in one physical space, for it is mainly the relativepositions of the object and our body that determine what sensations weshall derive from the object.

　　Now our sense-data are situated in our private spaces, either the space ofsight or the space of touch or such vaguer spaces as other senses may giveus. If, as science and common sense assume, there is one publicall-embracing physical space in which physical objects are, the relativepositions of physical objects in physical space must more or lesscorrespond to the relative positions of sense-data in our private spaces.There is no difficulty in supposing this to be the case. If we see on aroad one house nearer to us than another, our other senses will bear outthe view that it is nearer; for example, it will be reached sooner if wewalk along the road. Other people will agree that the house which looksnearer to us is nearer; the ordnance map will take the same view; and thuseverything points to a spatial relation between the houses correspondingto the relation between the sense-data which we see when we look at thehouses. Thus we may assume that there is a physical space in whichphysical objects have spatial relations corresponding to those which thecorresponding sense-data have in our private spaces. It is this physicalspace which is dealt with in geometry and assumed in physics andastronomy.

　　Assuming that there is physical space, and that it does thus correspond toprivate spaces, what can we know about it? We can know only what isrequired in order to secure the correspondence. That is to say, we canknow nothing of what it is like in itself, but we can know the sort ofarrangement of physical objects which results from their spatialrelations. We can know, for example, that the earth and moon and sun arein one straight line during an eclipse, though we cannot know what aphysical straight line is in itself, as we know the look of a straightline in our visual space. Thus we come to know much more about the relationsof distances in physical space than about the distances themselves; we mayknow that one distance is greater than another, or that it is along thesame straight line as the other, but we cannot have that immediateacquaintance with physical distances that we have with distances in ourprivate spaces, or with colours or sounds or other sense-data. We can knowall those things about physical space which a man born blind might knowthrough other people about the space of sight; but the kind of thingswhich a man born blind could never know about the space of sight we alsocannot know about physical space. We can know the properties of therelations required to preserve the correspondence with sense-data, but wecannot know the nature of the terms between which the relations hold.

　　With regard to time, our feeling of duration or of the lapse oftime is notoriously an unsafe guide as to the time that has elapsed by theclock. Times when we are bored or suffering pain pass slowly, times whenwe are agreeably occupied pass quickly, and times when we are sleepingpass almost as if they did not exist. Thus, in so far as time isconstituted by duration, there is the same necessity for distinguishing apublic and a private time as there was in the case of space. But in so faras time consists in an order of before and after, there is no needto make such a distinction; the time-order which events seem to have is,so far as we can see, the same as the time-order which they do have. Atany rate no reason can be given for supposing that the two orders are notthe same. The same is usually true of space: if a regiment of men aremarching along a road, the shape of the regiment will look different fromdifferent points of view, but the men will appear arranged in the sameorder from all points of view. Hence we regard the order as true also inphysical space, whereas the shape is only supposed to correspond to thephysical space so far as is required for the preservation of the order.

　　In saying that the time-order which events seem to have is the same as thetime-order which they really have, it is necessary to guard against apossible misunderstanding. It must not be supposed that the various statesof different physical objects have the same time-order as the sense-datawhich constitute the perceptions of those objects. Considered as physicalobjects, the thunder and lightning are simultaneous; that is to say, thelightning is simultaneous with the disturbance of the air in the placewhere the disturbance begins, namely, where the lightning is. But thesense-datum which we call hearing the thunder does not take place untilthe disturbance of the air has travelled as far as to where we are.Similarly, it takes about eight minutes for the sun's light to reach us;thus, when we see the sun we are seeing the sun of eight minutes ago. Sofar as our sense-data afford evidence as to the physical sun they affordevidence as to the physical sun of eight minutes ago; if the physical sunhad ceased to exist within the last eight minutes, that would make nodifference to the sense-data which we call 'seeing the sun'. This affordsa fresh illustration of the necessity of distinguishing between sense-dataand physical objects.

　　What we have found as regards space is much the same as what we find inrelation to the correspondence of the sense-data with their physicalcounterparts. If one object looks blue and another red, we may reasonablypresume that there is some corresponding difference between the physicalobjects; if two objects both look blue, we may presume a correspondingsimilarity. But we cannot hope to be acquainted directly with the qualityin the physical object which makes it look blue or red. Science tells usthat this quality is a certain sort of wave-motion, and this soundsfamiliar, because we think of wave-motions in the space we see. But thewave-motions must really be in physical space, with which we have nodirect acquaintance; thus the real wave-motions have not that familiaritywhich we might have supposed them to have. And what holds for colours isclosely similar to what holds for other sense-data. Thus we find that,although the relations of physical objects have all sorts ofknowable properties, derived from their correspondence with the relationsof sense-data, the physical objects themselves remain unknown in theirintrinsic nature, so far at least as can be discovered by means of thesenses. The question remains whether there is any other method ofdiscovering the intrinsic nature of physical objects.

　　The most natural, though not ultimately the most defensible, hypothesis toadopt in the first instance, at any rate as regards visual sense-data,would be that, though physical objects cannot, for the reasons we havebeen considering, be exactly like sense-data, yet they may be moreor less like. According to this view, physical objects will, for example,really have colours, and we might, by good luck, see an object as of thecolour it really is. The colour which an object seems to have at any givenmoment will in general be very similar, though not quite the same, frommany different points of view; we might thus suppose the 'real' colour tobe a sort of medium colour, intermediate between the various shades whichappear from the different points of view.

　　Such a theory is perhaps not capable of being definitely refuted, but itcan be shown to be groundless. To begin with, it is plain that the colourwe see depends only upon the nature of the light-waves that strike theeye, and is therefore modified by the medium intervening between us andthe object, as well as by the manner in which light is reflected from theobject in the direction of the eye. The intervening air alters coloursunless it is perfectly clear, and any strong reflection will alter themcompletely. Thus the colour we see is a result of the ray as it reachesthe eye, and not simply a property of the object from which the ray comes.Hence, also, provided certain waves reach the eye, we shall see a certaincolour, whether the object from which the waves start has any colour ornot. Thus it is quite gratuitous to suppose that physical objects havecolours, and therefore there is no justification for making such asupposition. Exactly similar arguments will apply to other sense-data.

　　It remains to ask whether there are any general philosophical argumentsenabling us to say that, if matter is real, it must be of such and such anature. As explained above, very many philosophers, perhaps most, haveheld that whatever is real must be in some sense mental, or at any ratethat whatever we can know anything about must be in some sense mental.Such philosophers are called 'idealists'. Idealists tell us that whatappears as matter is really something mental; namely, either (as Leibnizheld) more or less rudimentary minds, or (as Berkeley contended) ideas inthe minds which, as we should commonly say, 'perceive' the matter. Thusidealists deny the existence of matter as something intrinsicallydifferent from mind, though they do not deny that our sense-data are signsof something which exists independently of our private sensations. In thefollowing chapter we shall consider briefly the reasons—in myopinion fallacious—which idealists advance in favour of theirtheory.