in the course of a careful review, says of the principle of substitution, “Surely it is a great omission not to discuss whence we get this great principle itself; whether it is a pure law of the mind, or only an approximate lesson of experience; and if a pure product of the mind, whether there are any other products of the same kind, furnished by our knowing faculty itself.” Professor Robertson, in his very acute review,17 likewise objects to the want of psychological and philosophical analysis. “If the book really corresponded to its title, Mr. Jevons could hardly have passed so lightly over the question, which he does not omit to raise, concerning those undoubted principles of knowledge commonly called the Laws of Thought.... Everywhere, indeed, he appears least at ease when he touches on questions properly philosophical; nor is he satisfactory in his psychological references, as on pp. 4, 5, where he cannot commit himself to a statement without an accompaniment of ‘probably,’ ‘almost,’ or ‘hardly.’ Reservations are often very much in place, but there are fundamental questions on which it is proper to make up one’s mind.”
These remarks appear to me to be well founded, and I must state why it is that I have ventured to publish an extensive work on logic, without properly making up my mind as to the fundamental nature of the reasoning process. The fault after all is one of omission rather than of commission. It is open to me on a future occasion to supply the deficiency if I should ever feel able to undertake the task. But I do not conceive it to be an essential part of any treatise to enter into an ultimate analysis of its subject matter. Analyses must always end somewhere. There were good treatises on light which described the laws of the phenomenon correctly before it was known whether light consisted of undulations or of projected particles. Now we have treatises on the Undulatory Theory which are very valuable and satisfactory, although they leave us in almost complete doubt as to what the vibrating medium really is. So I think that, in the present day, we need a correct and scientific exhibition of the formal laws of thought, and of the forms of reasoning based on them, although we may not be able to enter into any complete analysis of the nature of