The understanding of man is extremely limited, and, therefore, all sensations resulting from its action must of necessity be also limited. In order, therefore, to give these sensations greater impulse, and with redoubled force to attract the will to good and restrain it from evil, both natures, the spiritual and the animal, are so intimately connected with each other that their modifications, being mutually interchanged, impart strength to one another. Hence arises a fundamental law of mixed natures, which, being reduced to its primary divisions, runs thus: the activities of the body correspond to the activities of the mind; that is, any overstraining of a mental activity is necessarily followed by an overstraining of certain bodily actions,--just as the equilibrium, or harmonious action, of the mental powers is associated with that of the bodily powers in perfect accord. Further: mental indolence induces indolence in the bodily actions; mental inaction causes them to cease altogether. Thus, as perfection is ever accompanied by pleasure, imperfection by the absence of pleasure, this law may be thus expressed: Mental pleasure is invariably attended by animal pleasure, mental pain by animal pain. [Complacency and Displacency perhaps more aptly express the meaning of Lust and Unlust, which we translate by pleasure and pain.] S 13.--Mental Pleasure furthers the Welfare of the Human Frame. Thus, a sensation which embraces within its range the whole spiritual being agitates in the same measure the whole framework of the organic body,--heart, veins and blood, muscles and nerves, all, from those mighty nerves that give to the heart its living impulse of motion down to the tiny and unimportant nerves by which hairs are attached to the skin, share equally its influence. Everything tends to a more violent motion. If the sensation be an agreeable one, all these parts will acquire a higher degree of harmonious activity; the heart's beat will be free, lively, uniform, the blood will flow unchecked, gently or with fiery speed, according as the affection is of a gentle or violent description; digestion, secretion, and excretion will follow their natural course; the excitable membranes will pliantly play in a gentle vapor-bath, and excitability as well as sensitiveness will increase. Therefore the condition of the greatest momentary mental pleasure is at the same time the condition of the greatest bodily well-being.