All acts of reasoning seem to me to be different cases of one uniform process which may perhaps be best described as the substitution of similars... The chief difficulty consists in showing that all the forms of the old logic, as well as the fundamental rules of mathematical reasoning, may be explained upon the same principle; and it is to this difficult task I have devoted the most attention.

Preface - The Substitution of Similars, The True Principles of Reasoning (1869)