There is one other relation of our subject to philosophic problems which I have purposely left for final consideration. Our study has consisted mainly in the psychological analysis of illusions supposed to be known or capable of being known as such. Now, the modern association school professes to be able to resolve some of the so-called intuitions of common sense into elements exactly similar to those into which we have here been resolving what are acknowledged by all as illusions. This fact would seem to point to a close connection between the scientific study of illusion and the particular view of these fundamental intuitions taken by one philosophic school. In order to see whether there is really this connection, we must reflect a little further on the nature of the method which we have been pursuing. I have already had occasion to rise the expression "scientific psychology," or psychology as a positive science, and the meaning of this expression must now be more carefully considered. As a positive science, psychology is limited to the function of analyzing mental states, and of tracing their origin in previous and more simple mental states. It has, strictly speaking, nothing to do with the question of the legitimacy or validity of any mental act. Take a percept, for example. Psychology can trace its parentage in sensation, the mode in which it has come by its contents in the laws of association. But by common consent, a percept implies a presentative apprehension of an object now present to sense. Is this valid or illusory? This question psychology, as science, does not attempt to answer. It would not, I conceive, answer it even if it were able to make out that the whole mental content in the percept can be traced back to elementary sensations and their combinations. For the fact that in the chemistry of mind elements may combine in perfectly new forms does not disprove that the forms thus arising, whether sentiments or quasi-cognitions, are invalid. Much less can psychology dispute the validity of a percept if it cannot be sure that the mind adds nothing to sensation and its grouping; that in the genesis of the perceptive state, with its intuition of something external and now present as object, nothing like a form of intelligence is superimposed on the elements of sensation, giving to the result of their coalescence the particular unity which we find. Whether psychology as a positive science can ever be sure of this: whether, that is to say, it can answer the question, "How do we come by the idea of object?" without assuming some particular philosophic or extra-scientific theory respecting the ultimate nature of mind, is a point which I purposely leave open.