In attempting to define our attitude to the mechanical theory of life, we have first of all to make sure that we have a right to take up a definite position at all. We should have less right, or perhaps none, if this theory of life were really of a purely “biological” nature, built up entirely from the expert knowledge and data which the biologist alone possesses. But the principles, assumptions, supplementary ideas and modes of expression along all the six lines we have discussed, the style and method according to which the hypothesis is constructed, the multitude of separate presuppositions with which it works, and indeed everything that helps to build up and knit the biological details into a scientific hypothesis, are the materials of rational synthesis in general, and as such are subject to general as well as to biological criticism. What is there, for instance, in Weismann’s ingenious biophor-theory that can be called specifically biological, and not borrowed from other parts of the scientific system? One advantage, indeed, the biologist always has in this matter, apart from his special knowledge; that is, the technical instinct, the power of scenting out, so to speak, and immediately feeling the importance of the facts pertaining to his own discipline. It is this that gives every specialist the advantage over the layman in dealing with the data of his own subject. This power of instinctively appraising facts, which develops in the course of all special work, can, for instance in hypotheses in the domain of history, transform small details, which to the layman seem trivial, into weighty arguments. Similarly it may be that the success of the mechanical interpretation in regard to isolated processes may make its validity for many other allied processes certain, even though there is no precise proof of this. But we cannot regard this as a final demonstration of the applicability of the mechanical theory, since the same technical instinct in other experts leads them to reject the whole hypothesis.