Before beginning the study of the cell it will be well for us to try to get a clear notion of the exact nature of the problems we are trying to solve. We wish to explain the activities of life phenomena in such a way as to make them intelligible through the application of natural forces. That these processes are fundamentally chemical ones is evident enough. A chemical oxidation of food lies at the basis of all vital activity, and it is thus through the action of chemical forces that the vital powers are furnished with their energy. But the real problem is what it is in the living machine that controls these chemical processes. Fat and starch may be oxidized in a chemist's test tubes, and will there liberate energy; but they do not, under these conditions, manifest vital phenomena. Proteid may be brought in contact with oxygen without any oxidation occurring, and even if it is oxidized no motion or assimilation or reproduction occurs under ordinary conditions. These phenomena occur only when the oxidation takes place _in the living machine_. Our problem is then to determine, if possible, what it is in the living machine that regulates the oxidations and other changes in such a way as to produce from them vital activities. Why is it that the oxidation of starch in the living machine gives rise to motion, growth, and reproduction, while if the oxidation occurs in the chemist's laboratory, or even in a bit of dead protoplasm, it simply gives rise to heat? One of the primary questions to demand attention in this search is whether we are to find the explanation, at the bottom, a _chemical_ or a _mechanical_ one. In the simplest form of life in which vital manifestations are found are we to attribute these properties simply to chemical forces of the living substance, or must we here too attribute them to the action of a complicated machinery? This question is more than a formal one. That it is one of most profound significance will appear from the following considerations: