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_.
Of a ruling Principle, or Law, we can only get a notion by observing the effects of certain things in relation to the mind; the uniformity of which leads us to infer something which is unchangeable and permanent. It is in this way that, either directly or indirectly, we learn the existence of certain laws that invariably control us. Thus, indirectly, from our disgust at monotony, we infer the necessity of variety. But variety, when carried to excess, results in weariness. Some limitation, therefore, seems no less needed. It is, however, obvious, that all attempts to fix the limit to Variety, that shall apply as a universal rule, must be nugatory, inasmuch as the _degree_ must depend on the _kind_, and the kind on the subject treated. For instance, if the subject be of a gay and light character, and the emotions intended to be excited of a similar nature, the variety may be carried to a far greater extent than in one of a graver character. In the celebrated Marriage at Cana, by Paul Veronese, we see it carried, perhaps, to its utmost limits; and to such an extent, that an hour's travel will hardly conduct us through all its parts; yet we feel no weariness throughout this journey, nay, we are quite unconscious of the time it has taken. It is no disparagement of this remarkable picture, if we consider the subject, not according to the title it bears, but as what the Artist has actually made it,--that is, as a Venetian entertainment; and also the effect intended, which was to delight by the exhibition of a gorgeous _pageant_. And in this he has succeeded to a degree unexampled; for literally the eye may be said to _dance_ through the picture, scarcely lighting on one part before it is drawn to another, and another, and another, as by a kind of witchery; while the subtile interlocking of each successive novelty leaves it no choice, but, seducing it onward, still keeps it in motion, till the giddy sense seems to call on the imagination to join in the revel; and every poetic temperament answers to the call, bringing visions of its own, that mingle with the painted crowd, exchanging forms, and giving them voice, like the creatures of a dream.