In still another way the psychologist is forced to be something of a nerve-physiologist. Mental phenomena are not only conditioned _a parte ante_ by bodily processes; but they lead to them _a parte post_. That they lead to _acts_ is of course the most familiar of truths, but I do not merely mean acts in the sense of voluntary and deliberate muscular performances. Mental states occasion also changes in the calibre of blood-vessels, or alteration in the heart-beats, or processes more subtle still, in glands and viscera. If these are taken into account, as well as acts which follow at some _remote period_ because the mental state was once there, it will be safe to lay down the general law that _no mental modification ever occurs which is not accompanied or followed by a bodily change_. The ideas and feelings, _e.g_., which these present printed characters excite in the reader's mind not only occasion movements of his eyes and nascent movements of articulation in him, but will some day make him speak, or take sides in a discussion, or give advice, or choose a book to read, differently from what would have been the case had they never impressed his retina. Our psychology must therefore take account not only of the conditions antecedent to mental states, but of their resultant consequences as well. * * * * * But actions originally prompted by conscious intelligence may grow so automatic by dint of habit as to be apparently unconsciously performed. Standing, walking, buttoning and unbuttoning, piano-playing, talking, even saying one's prayers, may be done when the mind is absorbed in other things. The performances of animal _instinct_ seem semi-automatic, and the _reflex acts_ of self-preservation certainly are so. Yet they resemble intelligent acts in bringing about the _same ends_ at which the animals' consciousness, on other occasions, deliberately aims. Shall the study of such machine-like yet purposive acts as these be included in Psychology?