Strangely enough, Hume finds himself face to face with what is in principle the same difficulty, and treats it in a not dissimilar way. "There is, however, one contradictory phenomenon, which may prove, that 'tis not absolutely impossible for ideas to go before their correspondent impressions. I believe it will readily be allow'd, that the several distinct ideas of colours, which enter by the eyes, or those of sounds, which are convey'd by the hearing, are really different from each other, tho' at the same time resembling. Now if this be true of different colours, it must be no less so of the different shades of the same colour, that each of them produces a distinct idea, independent of the rest. For if this shou'd be deny'd, 'tis possible, by the continual gradation of shades, to run a colour insensibly into what is most remote from it; and if you will not allow any of the means to be different, you cannot without absurdity deny the extremes to be the same. Suppose therefore a person to have enjoyed his sight for thirty years, and to have become perfectly well acquainted with colours of all kinds, excepting one particular shade of blue, for instance, which it never has been his fortune to meet with. Let all the different shades of that colour, except that single one, be plac'd before him, descending gradually from the deepest to the lightest; 'tis plain that he will perceive a blank, where that shade is wanting, and will be sensible, that there is a greater distance in that place betwixt the contiguous colours, than in any other. Now I ask, whether 'tis possible for him, from his own imagination, to supply this deficiency, and raise up to himself the idea of that particular shade, tho' it had never been conveyed to him by his senses? I believe there are few but will be of opinion that he can; and this may serve as a proof, that the simple ideas are not always derived from the correspondent impressions; tho' the instance is so particular and singular, that 'tis scarce worth our observing, and does not merit that for it alone we should alter our general maxim."[14]
FOOTNOTES
[1] p. 246.
[2] The assertion that all perceptions (i. e. all objects of perception) are extensive quantities relates, according to Kant, to the nature of objects, while the assertion that an event must have a necessary antecedent affirms that such an antecedent must exist, but gives no clue to its specific nature. Compare "But the existence of phenomena cannot be known a priori, and although we could be led in this way to infer the fact of some existence, we should not know this existence determinately, i. e. we could not anticipate the respect in which the empirical perception of it differed from that of other existences". (B. 221, M. 134). Kant seems to think that the fact that the dynamical principles relate to the existence of objects is a sufficient justification of their name.
It needs but little reflection to see that the distinctions which Kant draws between the mathematical and the dynamical principles must break down.
[3] B. 203-4, M. 123.
[4] B. 210, M. 127.
[5] Cf. pp. 37-9.
[6] The context shows that Kant is thinking only of such temporal relations as belong to the physical world, and not of those which belong to us as apprehending it. Cf. p. 139.
[7] B. 209-10, M. 127.