Page:Popular Science Monthly Volume 70.djvu/83

THE VALUE OF SCIENCE

By M. H. POINCARÉ

MEMBER OF THE INSTITUTE OF FRANCE

§ 3. Tactile Space

THUS I know how to recognize the identity of two points, the point occupied by $\mathrm {A}$ at the instant $\alpha$ and the point occupied by $\mathrm {B}$ at the instant $\beta$ but only on one condition, namely, that I have not budged between the instants $\alpha$ and $\beta$. That does not suffice for our object. Suppose, therefore, that I have moved in any manner in the interval between these two instants, how shall I know whether the point occupied by $\mathrm {A}$ at the instant a is identical with the point occupied by $\mathrm {B}$ at the instant $\beta$? I suppose that at the instant $\alpha$, the object $\mathrm {A}$ was in contact with my first finger and that in the same way, at the instant $\beta$, the object $\mathrm {B}$ touches this first finger; but at the same time, my muscular sense has told me that in the interval my body has moved. I have considered above two series of muscular sensations $S$ and $S'$, and I have said it sometimes happens that we are led to consider two such series $S$ and $S'$ as inverse one of the other, because we have often observed that when these two series succeed one another our primitive impressions are reestablished.

If then my muscular sense tells me that I have moved between the two instants $\alpha$ and $\beta$, but so as to feel successively the two series of muscular sensations $S$ and $S'$ that I consider inverses, I shall still conclude, just as if I had not budged, that the points occupied by $\mathrm {A}$ at the instant $\alpha$ and by $\mathrm {B}$ at the instant $\beta$ are identical, if I ascertain that my first finger touches $\mathrm {A}$ at the instant $\alpha$ and $\mathrm {B}$ at the instant $\beta$.

This solution is not yet completely satisfactory, as one will see. Let us see, in fact, how many dimensions it would make us attribute to space. I wish to compare the two points occupied by $\mathrm {A}$ and $\mathrm {B}$ at the instants $\alpha$ and $\beta$, or (what amounts to the same thing since I suppose that my finger touches $\mathrm {A}$ at the instant $\alpha$ and $\mathrm {B}$ at the instant $\beta$) I wish to compare the two points occupied by my finger at the two instants $\alpha$ and $\beta$. The sole means I use for this comparison is the series $\Sigma$ of muscular sensations which have accompanied the movements of my body between these two instants. The different imaginable series $\Sigma$ form evidently a physical continuum of which the number of dimensions is very great. Let us agree, as I have done, not to consider as distinct the two series $\Sigma$ and $\Sigma +s+s'$, when $S$ and $S'$ are inverses one of the other in the sense above given to this word; in spite of this