It is of the greatest importance in medical practice to distinguish between these different forms of muscular activity. There is a vast physiological difference between muscular contraction with the production of positive work, and muscular contraction without the production of work, or with negative work. To climb a flight of stairs is to contract the muscles with the production of work equal to the weight of the body multiplied by the height of the stairs. To descend the stairs is to contract the same muscles, but with the production of negative work, and consequently a maximum of heat. To walk on level ground is to contract the muscles with the production of little or no external work; as in a machine turning without friction in a vacuum. We have seen that a fall of potential and a current of energy are the necessary conditions for the production of any natural phenomenon. Hence we may assume that the phenomenon of sensation is also accompanied by a fall of potential and a current of energy. When we touch a hot body, there is a flow of energy from the hot body to the hand. When we touch a cold body, there is a current of energy in the opposite direction, {111} from the hand to the body. It was formerly held, and is still held by some physiologists, that the chief characteristic of life is the disproportion between an excitation and the response which it invokes from the organism. Such a doctrine can only be held by one who believes, at least implicitly, that the phenomena of life are supernatural, or at all events different in their nature from all other phenomena; for the disproportion between an excitation and the response it evokes is by no means confined to living things. This disproportion is universal in nature, and quite in conformity with the physical laws which govern the transformation of energy. The energy of living things is potential energy--a fact which has been too little recognized. In the case of reflex actions it is self-evident, because the response is immediate, and always the same for the same stimulus. As in all other transformations, the stimulus consists in the intervention of a minimal quantity of external energy. Long before the discovery of the laws of energy, Lamarck had recognized and formulated this fact. He writes: "What would vegetable life be without excitations from without, what would be the life even of the lower animals without this cause?" In another passage, seeking for a power capable of exciting the action of the organism, he says: "The lower animal forms, without nervous system, live only by the aid of excitations which they receive from without. In the lowest forms of life this exciting force is borrowed directly from the environment, while in the higher forms the external exciting force is transferred to the interior of the living being and placed at the disposal of the individual."