The father of mental philosophy, Aristotle, begins his work on ethics by telling us, that nothing exists without some theory or reason attached to it. The following out of this view leads to classification--that great engine of knowledge. We see things at first in isolated individuality or confused masses. Investigation teaches us to separate them into groups, which have some common and important principle of unity, though each individual of the group may be different from the others in detail. Thus we arrive at the great classifications of natural science, with which every one is more or less familiar. But the works of men have their classification too, for in human effort like causes produce like effects. Most people know what schools of poetry, painting, and music are. In architecture, we know, too, that there are great divisions--such as classic and Gothic. But many have yet to learn how far classification may go; and it is a new feature to have the peculiar national architecture of Scotland separated from that of England, and its peculiarities traced to interesting national events and habits. The common observer is apt to think that all buildings are much alike, or that each is alone in its peculiarities. Before classification can take place, there must be a collection and comparison of leading characteristics; and this is not easily accomplished with the edifices scattered over a whole country. It may be said that it was never done for Scotland, until Mr Billings completed his great series of engravings of the baronial and ecclesiastical antiquities of Scotland.