[13] Stumpf: Tonpsychologie, i. pp. 7, 8. Hobbes's phrase, _sentire semper idem et non sentire ad idem recidunt_, is generally treated as the original statement of the relativity doctrine. J. S. Mill (Examn. of Hamilton, p. 6) and Bain (Senses and Intellect, p. 321; Emotions and Will, pp. 550, 570-2; Logic, i. p. 2; Body and Mind, p. 81) are subscribers to this doctrine. Cf. also J. Mill's Analysis, J. S. Mill's edition, II. 11, 12. [14] We can steadily hear a note for half an hour. The differences between the senses are marked. Smell and taste seem soon to get fatigued. [15] In the popular mind it is mixed up with that entirely different doctrine of the 'Relativity of Knowledge' preached by Hamilton and Spencer. This doctrine says that our knowledge is relative _to us_, and is not of the object as the latter is in itself. It has nothing to do with the question which we have been discussing, of whether our objects of knowledge contain absolute terms or consist altogether of relations. [16] What follows in brackets, as far as p. 27, is from the pen of my friend and pupil Mr. E. B. Delabarre. [17] These phenomena have close analogues in the phenomena of contrast presented by the temperature-sense (see W. Preyer in Archiv f. d. ges. Phys., Bd. xxv. p. 79 ff.). Successive contrast here is shown in the fact that a warm sensation appears warmer if a cold one has just previously been experienced; and a cold one colder, if the preceding one was warm. If a finger which has been plunged in hot water, and another which has been in cold water, be both immersed in lukewarm water, the same water appears cold to the former finger and warm to the latter. In simultaneous contrast, a sensation of warmth on any part of the skin tends to induce the sensation of cold in its immediate neighborhood; and _vice versâ_. This may be seen if we press with the palm on two metal surfaces of about an inch and a half square and three-fourths inch apart; the skin between them appears distinctly warmer. So also a small object of exactly the temperature of the palm appears warm if a cold object, and cold if a warm object, touch the skin near it. [18] Helmholtz, Physiolog. Optik, p. 392. [19] _Loc. cit._ p. 407. [20] _Loc. cit._ p. 408. [21] _Loc. cit._ p. 406. [22] E. Hering, in Hermann's Handbuch d. Physiologie, iii. 1, p. 565.