It is true that certain æstheticians have attempted a distinction between stimuli and stimuli, between _natural_ and _conventional_ signs. The former are held to have a constant effect upon all; the latter only upon a limited circle. In their belief, signs employed in painting are natural, those used in poetry conventional. But the difference between them is at the most only one of degree. It has often been said that painting is a language understood by all, while with poetry it is otherwise. Here, for example, Leonardo found one of the prerogatives of his art, "which hath not need of interpreters of different tongues as have letters," and it pleases man and beast. He relates the anecdote of that portrait of the father of a family "which the little grandchildren were wont to caress while they were still in swaddling-clothes, and the dogs and cats of the house in like manner." But other anecdotes, such as those of the savages who took the portrait of a soldier for a boat, or considered the portrait of a man on horseback to be furnished with only one leg, are apt to shake one's faith in the understanding of painting by sucklings, dogs and cats. Fortunately, no arduous researches are necessary to convince oneself that pictures, poetry and all works of art only produce effects upon souls prepared to receive them. Natural signs do not exist; because all are equally conventional, or, to speak with greater exactness, _historically conditioned._ [Sidenote: _The surmounting of variety._] Granting this, how are we to succeed in causing the expression to be reproduced by means of the physical object? How obtain the same effect, when the conditions are no longer the same? Would it not, rather, seem necessary to conclude that expressions cannot be reproduced, despite the physical instruments made for the purpose, and that what is called reproduction consists in ever new expressions? Such would indeed be the conclusion if the varieties of physical and psychical conditions were intrinsically insurmountable. But since the insuperability has none of the characteristics of necessity we must on the contrary conclude that reproduction always occurs when we can replace ourselves in the conditions in which the stimulus (physical beauty) was produced. Not only can we replace ourselves in these conditions as an abstract possibility, but as a matter of fact we do so continually. Individual life, which is communion with ourselves (with our past), and social life, which is communion with our like, would not otherwise be possible. [Sidenote: _Restorations and historical interpretation._]