Now, the _actual_ human system approaches toward this _ideal_ one in a greater or less degree. It possesses all the faculties, both physical and psychical, but they are tainted with imperfections, and their health varies from the highest state attainable, down to some assumed standard, below which we say that a diseased condition exists. It will, however, be observed, and hereafter more definitely appear, that this border-line is merely one of assumption. No definite standard can be applied to all persons, and a condition which would be normal for one person may not be so for another. Besides, the _actual_ condition of many persons is one of changing stability, both in respect of the body and the mind, and this may depend upon causes which operate from either _within_ or _without_. A few simple statements in reference to differences existing among persons, as to those physical and mental conditions which are inherited, may be in point, and prepare the way for other considerations. I. Though we may not be able to determine the causes of these differences, yet it is quite evident that there exists, from the time of birth, the largest diversity in reference to the physical constitution of persons: while some are strong and vigorous, and capable of large effort, and of enduring exposure to the heat and cold with almost indifference, others are so delicate and sensitive as to be easily affected by such influences; while the muscles of some are susceptible of making the most delicate and difficult movements in all mechanical operations, with very little training or education, those of others can never be trained sufficiently to be able to accomplish them; the skin and the lungs of some persons are so constituted as to be easily influenced by such degrees of moisture and cold as have little or no such effect on those of others; the capacity for labor, and endurance, also varies very largely. Again, these physical conditions are more or less variable with many persons. They are conscious of feeling more active and vigorous, of experiencing a larger degree of pleasure in physical activity at one time than at another; they may be conscious of more or less painful sensations, experience a measure of indisposition to make effort; they may be more restless and uneasy, and feel discomfort from slight causes which had rarely before produced such results.