It is by observing what a thing becomes that we learn the part it may hereafter play in the general scheme of things, and come to know its real nature, and estimate it at its real value. Thus our estimate of the value of such an institution as "taboo" goes up, and our knowledge of it is increased, when we recognise in it one of the early manifestations of the sense of moral obligation on its negative side. Again, in tracing the evolution of religion it is impossible to know which of the various rites and ceremonies, practised by a savage tribe in its dealings with the supernatural, are religious and which non-religious, without taking into consideration the question, What do such customs tend to develop into? Until we know that, and until we can say whether what is evolved out of them is religious or non-religious--a question which we cannot answer unless we know what religion is--we cannot be said to understand the nature of the savage rites that we are studying. But it is not from the origins of art, religion, or morality that we shall gain the answer to the question what art, morality, or religion is; for the question must be answered before we can recognise the origins when we see them, and can only be answered by reference to the ideal, which is the test and final cause not only of progress, but of the real. The ideal is a principle both of thought and of action. As a principle of thought it is the test by which we determine whether any given movement is progressive or regressive, and whether any given thing is what it appears or is alleged to be. As a principle of action it is that for which we strive, the purpose with which we act, the cause of any progress that we make. If we are not prepared to maintain that everything which takes place is an advance upon what preceded, we require some test whereby to distinguish what is progress from what is not, and we admit that progress is a possibility which may or may not be realised, and it becomes of interest to inquire on what conditions its realisation depends.