Sciences are divided into two great classes, differing both as to the matters which they treat, and the sources from which they draw their premises. These are the physical and the mental, or, as they are sometimes called, the moral sciences. The proper subjects of the first are the properties of matter; those of the second are the sensations, faculties, and habits of the human mind. As we have no experience of mind separated from matter (perhaps indeed are incapable of conceiving its existence), and as the mind can act only through the body, even the more purely mental sciences are forced to take notice of matter; and many of them, such as the sciences which have been called æsthetic, those which account for the pleasure which we derive from beauty and sublimity, seem at first sight to treat of little except material objects. But they consider those objects merely with reference to their effects on the human mind. To classify and account for those effects as a part of the philosophy of mind is the purpose of the science, and it regards in matter only the qualities which produce them. On the other hand, a botanist in the description of plants cannot omit the qualities which render them agreeable or useful to man. Without doubt, to be pleased by the sight and smell of a rose is as much an attribute of the human mind as the form, colour, and other qualities which occasion that pleasure are attributes of the rose. But it is to the rose only that the botanist looks. He states that it is beautiful and odoriferous as a part of the description of the plant, not of that of the being to whom it is beautiful and odoriferous. The same difference separates arts, though the line is less clearly marked. For as every art must use material instruments, it is to a certain extent physical; and as every art aims at producing pleasure or preventing pain, it must be, to a certain extent, mental. Still, however, the difference exists. No one would call rhetoric a physical art, though its teacher must deliver precepts as to voice and gesture. No one would call agriculture a mental art, though a treatise on agriculture would be incomplete which did not compare the advantages and disadvantages of task-work and day-work,--a comparison involving wide and numerous moral considerations.