Professor Jevons suggests that it is lack of time and attention that prevents the scientific investigator from attaining to a clear conception of what is meant by scientific method. This has something to do with it, but I think we may also maintain that the work of the investigator and that of the critic are somewhat different in kind, and require somewhat different powers of mind. We find a parallel to this elsewhere. Both in literature and in art men may be in the best sense productive, and yet may be poor critics. We are often wofully disappointed when we attend a lecture on poetry by a poet, or one on painting by an artist. It may be said: If what is maintained above regarding the possibility of prosecuting scientific researches without having recourse to reflective thought is true, why should the man of science care whether the principles and methods of the non-philosophical sciences are investigated or are merely taken for granted? I answer: It should be observed that the statements made in the last section were somewhat guarded. I have used the expressions "as a rule" and "usually." I have spoken thus because one can work in the way described, without danger of error, only where a beaten track has been attained and is followed. In Chapter XVI it was pointed out that even in the mathematical sciences one may be forced to reflect upon the significance of one's symbols. As I write this, a pamphlet comes to hand which is concerned to prove that "every cause is potentially capable of producing several effects," and proves it by claiming that the square root of four ([square root symbol]4) is a _cause_ which may have as _effect_ either two (2) or minus two (-2). Is this mathematical reasoning? Are mathematical relations ever those of cause and effect? And may one on the basis of such reasonings claim that in nature the relation of cause and effect is not a fixed and invariable one? Even where there is a beaten track, there is some danger that men may wander from it. And on the confines of our knowledge there are fields in which the accepted road is yet to be established. Science makes constant use of hypotheses as an aid to investigation. What hypotheses may one frame, and what are inadmissible? How important an investigation of this question may be to the worker in certain branches of science will be clear to one who will read with attention Professor Poincare's brilliant little work on "Science and Hypothesis." [2]