Many distinguished thinkers, especially on the physiological side (Wundt and Ziehen, for instance) take the ideas of consciousness and psychic function to be identical--"all psychic action is conscious"; the province of psychic life, they say, is coextensive with that of consciousness. In our opinion, such a definition gives an undue extension to the meaning of consciousness, and occasions many errors and misunderstandings. We share, rather, the view of other philosophers (Romanes, Fritz Schultze, and Paulsen), that even our unconscious presentations, sensations, and volitions pertain to our psychic life; indeed, the province of these unconscious psychic actions (reflex action, and so forth) is far more extensive than that of consciousness. Moreover, the two provinces are intimately connected, and are separated by no sharp line of demarcation. An unconscious presentation may become conscious at any moment; let our attention be withdrawn from it by some other object, and forthwith it disappears from consciousness once more. The only source of our knowledge of consciousness is that faculty itself; that is the chief cause of the extraordinary difficulty of subjecting it to scientific research. Subject and object are one and the same in it: the perceptive subject mirrors itself in its own inner nature, which is to be the object of our inquiry. Thus we can never have a complete objective certainty of the consciousness of others; we can only proceed by a comparison of their psychic condition with our own. As long as this comparison is restricted to _normal_ people we are justified in drawing certain conclusions as to their consciousness, the validity of which is unchallenged. But when we pass on to consider _abnormal_ individuals (the genius, the eccentric, the stupid, or the insane) our conclusions from analogy are either unsafe or entirely erroneous. The same must be said with even greater truth when we attempt to compare human consciousness with that of the animals (even the higher, but especially the lower). In that case such grave difficulties arise that the views of physiologists and philosophers diverge as widely as the poles on the subject. We shall briefly enumerate the most important of these views.