The facts upon which the current theory is founded are well known, and, apparently, thoroughly established. Briefly, they are these: So much food, oxidised or burned outside the body, can be shown to yield so much heat and energy. The same foods, oxidised within the body, yield approximately the same amount of energy. Further, the energy which the body expends (in conscious and unconscious muscular activity, thought, emotion, and as heat, etc.) is, it is contended, practically equivalent to the energy which is thus supplied. There is, therefore, an equivalence, a balance, between income and outgo of energy: so that the recently conducted experiments in calorimetry are held to prove beyond question the causation of vital energy by food. I shall not in this place stop to question the accuracy of the figures obtained--to point out that the results do not always tally; that far too little allowance has been made for mental and emotional states, etc. I shall assume that the figures are accurate and prove all that they are held to prove. The question then arises: Do the figures prove the causation of vital energy by food? Apparently they do, no doubt, and they are held to do so by the majority of experimental physiologists; but I do not believe that this is at all the case. Admitting the facts, admitting far greater accuracy than the figures really show, we have to consider the question of their _interpretation_. And this brings us back to the remarks made at the beginning of this paper--that coincidence does not prove causation; and that the same set of facts may often be interpreted in an entirely different manner--one which would show that life is not directly dependent upon food combustion at all, as is generally supposed. The alternative method of interpreting the facts would be as follows: Life is a _power_ which acts upon organized matter, under certain conditions, in a variable and fluctuating manner. Whenever energy acts upon substance, substance wastes. Whenever work of any kind is done by the body, therefore, the tissues are broken down, and to supply this waste, this destruction, food material is needed. The more waste, the greater the need for repair, and _per contra_ the less waste, the less the need of repair. So far as the material equivalent (food) is concerned, therefore, it will be seen that this is only what we should expect on either theory; and tells no more in favour of one than the other.