Why did he give notice at the very beginning of his "General Principles" that he was going to speak about natural activities--firstly what they are, how they take place, and in what situations--and then, in the case of urinary secretion, declared that this took place through the kidneys, but left out its method of occurrence? It must, then, have been for no purpose that he told us how digestion occurs, or spends time upon the secretion of biliary superfluities;[149] for in these cases also it would have been sufficient to have named the parts through which the function takes place, and to have omitted the method. On the contrary, in these cases he was able to tell us not merely through what organs, but also in what way it occurs--as he also did, I think, in the case of _anadosis_; for he was not satisfied with saying that this took place through the veins, but he also considered fully the method, which he held to be from the tendency of a vacuum to become refilled. Concerning the secretion of urine, however, he writes that this occurs through the kidneys, but does not add in what _way_ it occurs. I do not think he could say that this was from the tendency of matter to fill a vacuum,[150] for, if this were so, nobody would have ever died of retention of urine, since no more can flow into a vacuum than has run out. For, if no other factor comes into operation[151] save only this tendency by which a vacuum becomes refilled, no more could ever flow in than had been evacuated. Nor could he suggest any other plausible cause, such, for example, as the expression of nutriment by the stomach[152] which occurs in the process of anadosis; this had been entirely disproved in the case of blood in the vena cava;[153] it is excluded, not merely owing to the long distance, but also from the fact that the overlying heart, at each diastole, robs the vena cava by violence of a considerable quantity of blood.