The area of thought, memory, and imagination has not been clearly located, except that these mental states are believed to have their seat in the _cortex_ or outer thin rind of gray brain matter which envelopes and covers the mass of brain substance. It is, moreover, considered probable that the higher processes of reasoning are performed in or by the cortex of the frontal lobes. The cortex of a person of average intelligence, if spread out on a flat surface, measures about four square feet. The higher the degree of intelligence possessed by a lower animal or human being, as a rule, the deeper and more numerous are the folds or convolutions of the cortex, and the finer its structure. It may be stated as a general rule, with but very few exceptions, that the higher the degree of intelligence in a lower animal or human being, the greater is the area of its cortex in proportion to the size of the brain. The cortex, it must be remembered, is folded into deep furrows or convolutions, the brain in shape, divisions, and convolutions resembling the inner portion of an English walnut. The interior of the two hemispheres of the cerebrum is composed largely of connective nerves which doubtless serve to produce and maintain the unity of function of the mental processes. While physiological psychology has performed great work in discovering brain-centers and explaining much of the mechanism of mental processes, it has but touched the most elementary and simple of the mental processes. The higher processes have so far defied analysis or explanation in the terms of physiology. CHAPTER IV. Consciousness. The fact of consciousness is the great mystery of psychology. It is difficult even to define the term, although every person of average intelligence understands what is sought to be conveyed by it. Webster defines it as "knowledge of one's own existence, sensations, mental operations, etc.; immediate knowledge or perception of any object, state, or sensation; being aware; being sensible of." Another authority defines the term as "the state of being aware of one's sensations; the power, faculty, or mental state of being aware of one's own existence, condition at the moment, thoughts, feelings, and actions." Halleck's definition is: "That indefinable characteristic of mental states which causes us to be aware of them."