Halleck has well said: "If a materialist should hold that the mind was nothing but the brain, and that the brain was a vast aggregation of molecular sheep herding together in various ways, his hypothesis would not change the fact that sensation must precede perception, memory and thought; nor would the laws of the association of ideas be changed, nor would the fact that interest and repetition aid memory cease to hold good. The man who thought his mind was a collection of little cells would dream, imagine, think and feel; so also would he who believed his mind to be immaterial. It is very fortunate that the same mental phenomena occur, no matter what theory is adopted. Those who like to study the puzzles as to what mind and matter really are must go to metaphysics. Should we ever find that salt, arsenic and all things else are the same substance with a different molecular arrangement, we should still not use them interchangeably." For the purposes of the study of practical psychology, we may as well lay aside, if even for the moment, our pet metaphysical conceptions and act as if we knew nothing of the essential nature of Mind (and indeed Science in truth does _not_ know), and confine ourselves to the phenomena and manifestations of Mind which, after all, is the only way in which and by which we can know anything at all about it. As Brooks says: "The mind can be defined only by its activities and manifestations. In order to obtain a definition of the mind, therefore, we must observe and determine its various forms of activity. These activities, classified under a few general heads and predicated of the unseen something which manifests them, will give us a definition of mind." The act of consciousness determines the existence of Mind in the person experiencing it. No one can be conscious of thought and, at the same time, deny the existence of mind within himself. For the very act of denial, in itself, is a manifestation of thought and consequently an assertion of the existence of mind. One may assert the axiom: "I think, therefore, I have a mind;" but he is denied the privilege of arguing: "I think, therefore, I have no mind." The mind has an ultimate and final knowledge of its own existence.