1. It is an old saying that we cannot understand life unless we understand death. The dead body, if its temperature is not too low and if it contains enough water, undergoes rapid disintegration. It was natural to argue that life is that which resists this tendency to disintegration. The older observers thought that the forces of nature determined the decay, while the vital force resisted it. This idea found its tersest expression in the definition of Bichat, that “life is the sum total of the forces which resist death.” Science is not the field of definitions, but of prediction and control. The problem is: first, how does it happen that as soon as respiration has ceased only for a few minutes the human body is dead, that is to say, will commence to undergo disintegration, and second, what protects the body against this decay while the respiration goes on, although temperature and moisture are such as to favour decay? The earlier biologists had already raised the question why it was that the stomach and intestine did not digest themselves. The hydrochloric acid and the pepsin in the stomach and the trypsin in the intestine digest proteins taken in in the form of food; why do they not digest the proteins of the cells of the stomach and the intestine? They will promptly digest the stomach as soon as the individual is dead, but not during life. A self-digestion may also be caused if the arteries of the stomach are ligatured. Claude Bernard and others suggested that the layer of mucus protected the cells of the stomach and of the intestine from the digestive enzymes; or that the epithelial layer had a protective effect. Pavy suggested that the alkali of the blood had a protective action. All these theories became untenable when Fermi showed that all kinds of living organisms, protozoans, worms, arthropods, are not digested in solutions of trypsin as long as they are alive, while they are promptly digested in the same solution when dead.[294] This is in harmony with the fact that many parasites live in the intestine without being digested as long as they are alive. Fermi concluded that the living cell cannot be attacked by the digestive ferments, while with death a change occurs by which they can be attacked. But what is this change? Fermi seems to be inclined to think that the “living molecule” of protein is not hydrolysable (perhaps because the enzyme cannot attach itself to it?), while a change in the constitution or configuration of the proteins takes place after respiration has ceased. The fact that the living cell resists the digestive action of trypsin and pepsin has found two other modes of explanation, first, that the cells are surrounded by a membrane or envelope through which the enzyme cannot diffuse, and second, that the living cells possess antiferments. But the so-called antiferments are also said to exist after the death of the cell, whereas after death the cell is promptly digested. Frédéricq, as well as Klug, has shown