The ancients classed individuals in one or other of four temperaments, founded on the hypothesis of four humors, of which the blood was supposed to be composed--the red part, phlegm, yellow and black bile. These were regarded as the elements of the body, and their respective predominance passed for the cause of the differences which it presented. Hence were derived the names of the sanguine, the phlegmatic, the choleric, and the melancholic temperament. Although the hypothesis on which this doctrine was founded is universally discarded, the phenomena which observation had taught the ancients, and which they had hypothetically connected with these elements, were so true, that that classification has been more or less employed in all the hypotheses which have since been invented to explain their cause; and their nomenclature has continued in use to the present day. A temperament may be defined a peculiar state of the system, depending on the relative proportion of its different masses, and the relative energy of its different functions, by which it acquires a tendency to certain actions. The predominance of any particular organ or system of organs, its excess of force, extends its sphere of activity to all the other functions, and modifies them in a peculiar manner. Thus, conforming in the illustration to the preceding arrangement, in one person, the muscles are more frequently employed than the brain; in another, the stomach or the organs of reproduction are more employed than the muscles; and in a third, the brain and nerves are more employed than either. This predominance or excess establishes the temperament. The relative feebleness of any organ or system of organs, similarly forms modifications not less important. Thus in one person, the organs of the abdomen are less employed; in another, those of the chest; in a third, the brain. Disease, it is observed, "commonly enters into the organization by these feeble points: death even attacks them first; extends afterward from one to another; and makes progress more or less rapid, according to the importance of the organ primitively affected." Temperaments, however, vary infinitely. It may be said that every individual has a peculiar one, to which he owes his mode of existence and his degree of health, ability, and happiness. The temperament, moreover, of each individual is not always characterized by well-marked symptoms; and even where it has been strongly marked by nature, education, age, the influence of climate, the exercise of professions and trades, and various habits, produce in it infinite changes. Temperaments also combine together, so that all men are, in some degree, at once sanguine and bilious, or otherwise compound. Thus all intermediate shades of temperament are produced; and it is often difficult, or, under particular circumstances, impossible, to determine under which temperament individuals may be classed.