o′a′b′c′d′e′f′g′h′, and two triangles α β γ and α′ β′ γ′. The first double pyramid would be rectilinear, and the second curvilinear. The triangle α β γ would consist of undilatable matter, and the other of very dilatable matter. We might therefore make our first observations with the double pyramid o′a′h′ and the triangle α′ β′ γ′.
And then the experiment would seem to show—first, that Euclidean geometry is true, and then that it is false. Hence, experiments have reference not to space but to bodies.
8. To round the matter off, I ought to speak of a very delicate question, which will require considerable development; but I shall confine myself to summing up what I have written in the Revue de Métaphysique et de Morale and in the Monist. When we say that space has three dimensions, what do we mean? We have seen the importance of these "internal changes" which are revealed to us by our muscular sensations. They may serve to characterise the different attitudes of our body. Let us take arbitrarily as our origin one of these attitudes, A. When we pass from this initial attitude to another attitude B we experience a series of muscular sensations, and this series S of muscular sensations will define B. Observe, however, that we shall often look upon two series S and S′ as defining the same attitude B (since the
