in the form of a series of kinæsthetic sensations in his somatic sense-history. And it is reasonable to think that the other is of the same general nature. (ii) That this observer might reasonably hold that a certain identity between such processes in two physical objects A and B involves relative rest between them, and that differences between the two processes involve relative motion between A and B.

Now these conclusions, which are rendered highly plausible by the mere correlations between a solitary observer's kinæsthetic sensations and the motion or rest of his sense-objects, are greatly strengthened when the physical objects which he watches are the bodies. of other observers who can communicate with him.

(1) Suppose that observer a watches B, the body of observer B, and that at the same time observer B watches A, the body of observer a. The correlations between the kinæsthetic sensations and the visual senseobjects of each observer are of exactly the same kind as if he were watching an inorganic body. But, in the present case, the observer and the observed can compare notes about their kinæsthetic sensations and their visual sense-objects. Let us first suppose that a does not have to keep turning his head in order to keep his eye on B, and that B appears to him as a completely uniform visual sense-object. Then B will tell a that he, too, does not need to keep turning his head in order to keep his eye on A, and that A appears in his sensehistory as a completely uniform visual sense-object. If they now compare their translatory kinæsthetic sensations, they will find either that they are absent in both, or, if present, are of precisely the same character.

Let us next suppose that a finds that he has to keep turning his head in order to keep his eye on B. B will then appear in a's sense-history as a partly, but only partly, uniform sense-object. The nature of its nonuniformity has already been fully described. Now B will also find, and will tell a that he finds, that he must

keep turning his head in order to keep his eye on A, and that A appears in his sense-history as a partly, but only partly, uniform sense-object of the kind already described. In this case a and B will find, on comparing notes, that they both experience a series of rotational kinæsthetic sensations, and that there is an analogy between them. But, on the other hand, they will always find that there is a difference between their translatory kinæsthetic sensations. This will sometimes take the form that one and only one of them has such sensations at all (I am leaving out of account for the sake of simplicity observers who are carried about without effort in trains or motor-cars). There is one other important point which they will discover on comparing their experiences. The appearance of a's head in B's sense-history will be a rotating visual senseobject, and so will be the appearance of B's head in a's sense-history. Thus each will discover that, of his two kinds of kinæsthetic sensation, one is correlated with a rotationally non-uniform sense-object by which his head appears in the sense-history of the other observer, and the other kind is correlated with a positionally nonuniform sense-object, by which his body appears in the sense-history of the other observer.

(2) So far, we have confined ourselves to two observers a and ẞ respectively watching B and A, the bodies of the other. Let us now take an observer y, who watches the bodies A and B of the two observers a and B, who can communicate with him and with each other. As we have said before, if y keeps up a suitable series of kinæsthetic sensations, he can always make A appear in his sense-history as a completely uniform sense-object, each of whose successive constituent sensa is at the middle of its field. We will suppose that y does this. He may then find either (i) that B appears as a completely uniform sense-object, or (ii) that B appears as a positionally non-uniform sense-object. Each of the component sensa in this may have sensible movement

in their fields. And, even if they do not, successive pairs of contemporary A- and B-sensa will have progressively different sensible distances in their respective common fields in y's visual sense-history.

Now, in case (i), a and ẞ will tell y that, on comparing notes with each other, they find no difference in their translational kinæsthetic sensations, which may, of course, in the limiting case both be non-existent. In case (ii), a and B will tell y that, on comparing notes, they do find a difference in their translational kinesthetic sensations. If one of them has no such sensations the other will have them. Moreover, each of them will tell γ that the body of the other appears to himself as a nonuniform sense-object. And y's body C will appear in B's, though not in a's, sense-history as a non-uniform sense-object.

Now these communicated experiences (1) and (2) leave no doubt at all that the positional uniformity or non-uniformity of the sense-object, by which one human body appears in the sense-history of another observer, depends jointly on those physical processes in the two bodies which are revealed to their respective minds in the form of kinæsthetic sensations. Moreover, they show clearly that uniformity in the sense-object depends on a certain identity of quality and quantity in the two processes, whilst positional non-uniformity in the senseobject depends on certain qualitative and quantitative differences between the two processes. Lastly (2) shows that relative motion of the sense-objects by which two human bodies appear in the sense-history of a third observer depends on a difference between these two processes in the two human bodies, whilst relative rest of two such sense-objects depends on an identity of character between the two processes.

We now extend this conclusion in the usual way to physical objects which are not connected with minds that can communicate with us. We assume that, in all cases, the uniformity of a sense-object in the sense

history of an observer depends upon a certain identity between that physical process in his own body which appears to him as a series of kinæsthetic sensations, and another physical process of the same general type, which happens in the physical object of which this uniform sense-object is the visual appearance in the observer's sense-history. And we assume that, in all cases, the positional non-uniformity of a sense-object in the sense-history of an observer depends on differences between the physical process in his body which appears to him as a series of kinæsthetic sensations, and another physical process of the same general type, which happens in the physical object of which this non-uniform senseobject is the visual appearance in this observer's sensehistory.

(e) Several Intercommunicating Observers watching the same Physical Object.-One more very important fact remains to be described. Suppose that two observers, a and B, are watching a certain physical object O, and that a third observer y is watching their bodies, A and B. It may happen that O appears in a's sense-history as an uniform sense-object, and that it appears in B's sense-history as a positionally non-uniform sense-object. If this be so, the observer y will always notice that the sense-objects by which A and B appear in his sensehistory are in relative motion to each other. And, as usual under these conditions, there will be a difference in the translational kinæsthetic sensations of a and B. If we generalise this from human bodies to all physical objects we reach the following conclusion: It is possible for any physical object to appear at once as a uniform sense-object in the sense-history of one observer and as a non-uniform sense-object in that of another observer. But, if it does so, it will always be found that there is some difference between those physical processes in the bodies of the two observers which appear to them as series of their kinæsthetic sensations.

This result, which can actually be observed, might

also have been deduced from what has gone before. If the physical object O appears as a resting sense-object in a's visual sense-history, this implies a certain identity of character between the relevant physical processes in A and in O, according to the argument of the last subsection. If O appears as a moving sense-object in B's sense-history, this implies a difference between the relevant physical processes in B and in O, on the same principles. It follows at once that, under these circumstances, there must be a difference between the relevant physical processes in A and in B. And this should appear to a and to ẞ as a difference between their kinæsthetic sensations. That such a difference is actually found supports the conclusions of the last sub-section, since they are here used as hypothetical premises from which it follows that such a difference ought to be found.

In the next chapter I propose to apply the results of this one to the notions of sensible and physical SpaceTime, and so to end my treatment of the spatio-temporal aspects of Nature and their sensible and perceptual basis.

The following additional works may be consulted with advantage:

G. F. STOUT, Manual of Psychology, Bk. III. Part II.
W. JAMES, Principles of Psychology.

KANT, Critique of Pure Reason (Analytic of Principles).
SCHOPENHAUER, World as Will and Idea, Vol. I. Bk. II.