It must be admitted that no psychologist is willing to stop with the _known_ and _proved_, but, when he has presented that, dips into the fascinations of the yet unknown, and works with promising theory, which tomorrow may prove to be science also. But we will first find what they have verified, and make that the safe foundation for our own understanding of ourselves and others. What do we mean by "mental life"?--or, we might say, the science of the life of the mind. And what is _mind_? But let us start our quest by asking first what reasons we have for being sure mind exists. We find the proof of it in consciousness, although we shall learn later that the activities of the mind may at times be unconscious. So where consciousness is, we know there is mind; but where consciousness is not, we must find whether it has been, and is only temporarily withdrawn, before we say "Mind is not here." And _consciousness_ we might call _awareness_, or our personal recognition of being--awareness of me, and thee, and it. So we recognize _mind_ by its evidences of awareness, _i. e._, by the body's reaction to stimuli; and we find mind at the very dawn of animal life. Consciousness is evidenced in the protozoön, the simplest form in which animal life is known to exist, by what we call its response to stimuli. The protozoön has a limited power of self-movement, and will accept or reject certain environments. But while we see that mind expresses itself in consciousness as vague, as dubious as that of the protozoön, we find it also as clear, as definite, as far reaching as that of the statesman, the chemist, the philosopher. Hence, the "phenomena of mental life" embrace the entire realms of feeling, knowing, willing--not of man alone, but of all creatures. In our study, however, we shall limit ourselves to the psychology of the human mind, since that concerns us vitally as nurses. Animal psychology, race psychology, comparative psychology are not within the realm of our practical needs in hospital life. We would know the workings of man's mind in disease and health. What are the instinctive responses to fear, as shown by babies and children and primitive races? What are the normal expressions of joy, of anger, or desire? What external conditions call forth these evidences? What are the acquired responses to the things which originally caused fear, or joy, or anger? How do grown-ups differ in their reactions to the same stimuli? Why do they differ? Why does one man walk firmly, with stern, set face, to meet danger? Why does another quake and run? Why does a third man approach it with a swagger, face it with a confident, reckless smile of defiance?