In a true Composition of Art will be found the following characteristics:--First, Unity of Purpose, as expressing the general sentiment or intention of the Artist. Secondly, Variety of Parts, as expressed in the diversity of shape, quantity, and line. Thirdly, Continuity, as expressed by the connection of parts with each other, and their relation to the whole. Fourthly, Harmony of Parts. As these characteristics, like every thing which the mind can recognize as true, all have their origin in its natural desires, they may also be termed Principles; and as such we shall consider them. In order, however, to satisfy ourselves that they are truly such, and not arbitrary assumptions, or the traditional dogmas of Practice, it may be well to inquire whence is their authority; for, though the ultimate cause of pleasure and pain may ever remain to us a mystery, yet it is not so with their intermediate causes, or the steps that lead to them. With respect to Unity of Purpose, it is sufficient to observe, that, where the attention is at the same time claimed by two objects, having each a different end, they must of necessity break in upon that free state required of the mind in order to receive a full impression from either. It is needless to add, that such conflicting claims cannot, under any circumstances, be rendered agreeable. And yet this most obvious requirement of the mind has sometimes been violated by great Artists,--though not of authority in this particular, as we shall endeavour to show in another place. We proceed, meanwhile, to the second principle, namely, Variety; by which is to be understood _difference_, yet with _relation_ to a _common end_.