The effect of music upon the mind, so penetrating, so direct, so unfailing, may be explained from the _passive_ nature of hearing which has been discussed; also the after effect which sometimes follows it, and which consists in a specially elevated frame of mind. The vibrations of the tones following in rationally combined numerical relations set the fibre of the brain itself in similar vibration. On the other hand, the _active_ nature of sight, opposed as it is to the passive nature of hearing, makes it intelligible why there can be nothing analogous to music for the eye, and the piano of colours was an absurd mistake. Further, it is just on account of the active nature of the sense of sight that it is remarkably acute in the case of beasts that hunt, _i.e._, beasts of prey, while conversely the _passive_ sense of hearing is specially acute in those beasts that are hunted, that flee, and are timid, so that it may give them timely warning of the pursuer that is rushing or creeping upon them.
Just as we have recognised in sight the sense of the understanding, and in hearing the sense of the reason, so we might call smell the sense of the memory, because it recalls to us more directly than any other the specific impression of an event or a scene even from the most distant past.
From the fact that we are able spontaneously to assign and determine the laws of relations in space without having recourse to experience, Plato concludes (_Meno_, p. 353, Bip.) that all learning is mere recollection. Kant, on the other hand, concludes that space is subjectively conditioned, and merely a form of the faculty of knowledge. How far, in this regard, does Kant stand above Plato!
_Cogito, ergo sum_, is an analytical judgment. Indeed Parmenides held it to be an identical judgment: “το γαρ αυτο νοειν εστι τε και ειναι” (_nam intelligere et esse idem est_, _Clem. Alex. Strom._, vi. 2, § 23). As such, however, or indeed even as an analytical judgment, it cannot contain any special wisdom; nor yet if, to go still deeper, we seek to deduce it as a conclusion from the major premise, _non-entis nulla sunt prædicata_. But with this proposition what Descartes really wished to express was the great truth that immediate certainty belongs only to self-consciousness, to what is subjective. To what is objective, on the other hand, thus to everything else, only indirect certainty belongs; for it is arrived at through self-consciousness; and being thus merely at second hand, it is to be regarded as problematical. Upon this depends the value of this celebrated proposition. As its opposite we may set up, in the sense of the Kantian philosophy, _cogito, ergo est_, that is, exactly as I think certain relations in things (the mathematical), they must always occur in all possible experience;—this was an important, profound, and a late _apperçu_, which appeared in the form of the problem as to the _possibility of synthetic judgments a priori_, and has actually opened up the way to a deeper knowledge. This problem is the watchword of the Kantian philosophy, as the former proposition is that of the Cartesian, and shows εξ οἱων εισ οἱα.