IX. THE DEFINITION OF MATTER

We defined the " physical thing ' as the class of its appearances, but this can hardly be taken as a definition of matter. We want to be able to express the fact that
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the appearance of a thing in a given perspective is causally affected by the matter between the thing and the perspective. We have found a meaning for " between a thing and a perspective." But we want matter to be something other than the whole class of appearances of a thing, in order to state the influence of matter on appear- ances.
We commonly assume that the information we get about a thing is more accurate when the thing is nearer. Far off, we see it is a man ; then we see it is Jones ; then we see he is smiling. Complete accuracy would only be attainable as a limit : if the appearances of Jones as we approach him tend towards a limit, that limit may be taken to be what Jones really is. It is obvious that from the point of view of physics the appearances of a thing close to " count " more than the appearances far off. We may therefore set up the following tentative definition :
The matter of a given thing is the limit of its appear- ances as their distance from the thing diminishes.
It seems probable that there is something in this definition, but it is not quite satisfactory, because em- pirically there is no such limit to be obtained from sense- data. The definition will have to be eked out by con- structions and definitions. But probably it suggests the right direction in which to look.
We are now in a position to understand in outline the reverse journey from matter to sense-data which is per- formed by physics. The appearance of a thing in a given perspective is a function of the matter composing the thing and of the intervening matter. The appearance of a thing is altered by intervening smoke or mist, by blue spectacles or by alterations in the sense-organs or nerves of the percipient (which also must be reckoned as part of the intervening medium). The nearer we approach to
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the thing, the less its appearance is affected by the inter- vening matter. As we travel further and further from the thing, its appearances diverge more and more from their initial character ; and the causal laws of their divergence are to be stated in terms of the matter which lies between them and the thing. Since the appearances at very small distances are less affected by causes other than the thing itself, we come to think that the limit towards which these appearances tend as the distance diminishes is what the thing " really is/' as opposed to what it merely seems to be. This, together with its necessity for the statement of causal laws, seems to be the source of the entirely erro- neous feeling that matter is more " real ' than sense- data.
Consider for example the infinite divisibility of matter In looking at a given thing and approaching it, one sense- datum will become several, and each of these will again divide. Thus one appearance may represent many things, and to this process there seems no end. Hence in the limit, when we approach indefinitely near to the thing there will be an indefinite number of units of mattti corresponding to what, at a finite distance, is only one appearance. This is how infinite divisibility arises.
The whole causal efficacy of a thing resides in its matter. This is in some sense an empirical fact, but it would be hard to state it precisely, because " causal efficacy " is difficult to define.
What can be known empirically about the matter of a thing is only approximate, because we cannot get to know the appearances of the thing from very small distances, and cannot accurately infer the limit of these appearances. But it is inferred approximately by means of the appear- ances we can observe. It then turns out that these appearances can be exhibited by physics as a function of
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the matter in our immediate neighbourhood ; e.g. the visual appearance of a distant object is a function of the light-waves that reach the eyes. This leads to confusions of thought, but offers no real difficulty.
One appearance, of a visible object for example, is not sufficient to determine its other simultaneous appearances, although it goes a certain distance towards determining them. The determination of the hidden structure of a thing, so far as it is possible at all, can only be effected by means of elaborate dynamical inferences.