But the confusion between Physic and Aesthetic has attained to its highest degree, when aesthetic theories of the different arts are imagined, to answer such questions as: What are the _limits_ of each art? What can be represented with colours, and what with sounds? What with simple monochromatic lines, and what with touches of various colours? What with notes, and what with metres and rhymes? What are the limits between the figurative and the auditional arts, between painting and sculpture, poetry and music? This, translated into scientific language, is tantamount to asking: What is the connexion between Acoustic and aesthetic expression? What between the latter and Optic?--and the like. Now, if _there is no passage_ from the physical fact to the aesthetic, how could there be from the aesthetic to particular groups of aesthetic facts, such as the phenomena of Optic or of Acoustic? [Sidenote] _Critique of the classifications of the arts._ The things called _Arts_ have no aesthetic limits, because, in order to have them, they would need to have also aesthetic existence; and we have demonstrated the altogether empirical genesis of those divisions. Consequently, any attempt at an aesthetic classification of the arts is absurd. If they be without limits, they are not exactly determinable, and consequently cannot be philosophically classified. All the books dealing with classifications and systems of the arts could be burned without any loss whatever. (We say this with the utmost respect to the writers who have expended their labours upon them.) The impossibility of such classifications finds, as it were, its proof in the strange methods to which recourse has been had to carry them out. The first and most common classification is that into arts of _hearing, sight_, and _imagination_; as if eyes, ears, and imagination were on the same level, and could be deduced from the same logical variable, as foundation of the division. Others have proposed the division into arts of _space and time_, and arts of _rest_ and _motion_; as if the concepts of space, time, rest, and motion could determine special aesthetic forms, or have anything in common with art as such. Finally, others have amused themselves by dividing them into _classic and romantic_, or into _oriental, classic, and romantic_, thereby conferring the value of scientific concepts on simple historical denominations, or adopting those pretended partitions of expressive forms, already criticized above; or by talking of arts _that can only be seen from one side_, like painting, and of arts _that can be seen from all sides_, like sculpture--and similar extravagances, which exist neither in heaven nor on the earth.