In the first place, the nature of the subject investigated would necessarily tend to such a result; for that subject is the human mind, not in its specific peculiarities, but in those generic phenomena which are common to all minds; just as the natural philosopher investigates those properties of matter which are common to a class, and not the specific peculiarities that distinguish individual masses or particles. Now, as those who direct their investigations to mental phenomena are all drawing a picture from the same pattern, it is properly inferred that in the main outlines there must be a general resemblance. Another reason for this conclusion is the mode of investigation pursued. It is simply observing, first, the phenomena of our own minds, and then comparing them with those of other minds as exhibited in looks, words, and actions, and thus educing generic resemblances and specific differences. It is the generic resemblances only that constitute the faculties and laws of mind which are to be described, classified, and named. Another reason for inferring such an agreement of systems is the fact, not only that all human minds have common phenomena, but that they have provided themselves with terms to express them, so that they succeed in so far understanding each other as to make comparisons of their mental experience. The same agreement may be inferred, also, when we consider that mental philosophy treats, not of new ideas, or new combinations of ideas, but of knowledge which is already in the mind. The process to be pursued, then, involves a reference to what we have ourselves experienced; it is an examination of our own feelings, thoughts, and volitions. These are subjects of which we are competent judges, and in regard to which we can be certain as to what is correct or incorrect, more than we can be in reference to any other kind of knowledge. From these considerations, it is inferred that all systems of mental philosophy will resemble each other just so far as they are true, and that the difference must be mainly in modes of presenting the subject. Inasmuch as writers on mental science are drawing a picture of those experiences of their own minds which are common to the whole race, they must in the main resemble each other, though some may be more imperfect, vague, and disconnected than others. It may be useful to indicate the causes which have combined to produce perplexity and apparent diversities among writers on mental science.