[Footnote 4: J. S. MILL, _An Examination of Sir Wm. Hamilton's Philosophy_, pp. 5 and 6. London. 1865.] [Footnote 5: A few subtle philosophers have returned to it, as I shall show later in chapter iv.] [Footnote 6: Thus, the perplexity in which John Stuart Mill finds himself is very curious. Having admitted unreservedly that our knowledge is confined to sensations, he is powerless to set up a reality outside this, and acknowledges that the principle of causality cannot legitimately be used to prove that our sensations have a cause which is not a sensation, because this principle cannot be applied outside the world of phenomena.] [Footnote 7: See p. 18, _sup_.--ED.] CHAPTER III THE MECHANICAL THEORIES OF MATTER ARE ONLY SYMBOLS If we keep firmly in mind the preceding conclusion--a conclusion which is neither exclusively my own, nor very new--we shall find a certain satisfaction in watching the discussions of physicists on the essence of matter, on the nature of force and of energy, and on the relations of ponderable and imponderable matter. We all know how hot is the fight raging on this question. At the present time it is increasing in intensity, in consequence of the disturbance imported into existing theories by the new discoveries of radio-activity.[8] We psychologists can look on very calmly at these discussions, with that selfish pleasure we unavowedly feel when we see people fighting while ourselves safe from knocks. We have, in fact, the feeling that, come what may from the discussions on the essence of matter, there can be no going beyond the truth that matter is an excitant of our nervous system, and is only known in connection with, the perception we have of this last. If we open a work on physics or physiology we shall note with astonishment how the above considerations are misunderstood. Observers of nature who seek, and rightly, to give the maximum of exactness to their observations, show that they are obsessed by one constant prejudice: they mistrust sensation. A great part of their efforts consists, by what they say, in reducing the role of sensation to its fitting part in science; and the invention of mechanical aids to observation is constantly held up as a means of remedying the imperfection of our senses. In physics the thermometer replaces the sensation of heat that our skin--our hand, for example--experiences by the measurable elevation of a column of mercury, and the scale-pan of a precise balance takes the place of the vague sensation of trifling weights; in physiology a registering apparatus replaces the sensation of the pulse which the doctor feels with the end of his forefinger by a line on paper traced with indelible ink, of which the duration and the intensity, as well as the varied combinations of these two elements, can be measured line by line.