"What is so clearly true of the nervous apparatus of animal life can scarcely be otherwise than true of that which ministers to the automatic activity of the mind. For, as already shown, the study of psychology has evolved no more certain result than that there are uniformities of mental action which are so entirely conformable to those of bodily action as to indicate their intimate relation to a 'mechanism of thought and feeling,' acting under the like conditions with that of sense and motion. The psychical principles of _association_, indeed, and the physiological principles of _nutrition_, simply express--the former in terms of mind, the latter in terms of brain--the universally admitted fact that any sequence of mental action which has been frequently repeated tends to perpetuate itself; so that we find ourselves automatically prompted to _think, feel,_ or _do_ what we have been before accustomed to think, feel, or do, under like circumstances, without any consciously formed _purpose_, or anticipation of results. For there is no reason to regard the cerebrum as an exception to the general principle that, while each part of the organism tends to _form itself_ in accordance with the mode in which it is habitually exercised, this tendency will be especially strong in the nervous apparatus, in virtue of that _incessant regeneration_ which is the very condition of its functional activity. It scarcely, indeed, admits of doubt that every state of ideational consciousness which is either _very strong_ or is _habitually repeated_ leaves an organic impression on the cerebrum; in virtue of which that same state may be reproduced at any future time, in respondence to a suggestion fitted to excite it.... The 'strength of early association' is a fact so universally recognized that the expression of it has become proverbial; and this precisely accords with the physiological principle that, during the period of growth and development, the formative activity of the brain will be most amenable to directing influences. It is in this way that what is early 'learned by heart' becomes branded in (as it were) upon the cerebrum; so that its 'traces' are never lost, even though the conscious memory of it may have completely faded out. For, when the organic modification has been once _fixed_ in the growing brain, it becomes a part of the normal fabric, and is regularly _maintained_ by nutritive substitution; so that it may endure to the end of life, like the scar of a wound." Dr. Carpenter's phrase that _our nervous system grows to the modes in which it has been exercised_ expresses the philosophy of habit in a nutshell. We may now trace some of the practical applications of the principle to human life. The first result of it is that _habit simplifies the movements required to achieve a given result, makes them more accurate and diminishes fatigue_.