  The absence of a clear definition of ‘identity’ is striking, especially since Jevons recognises that there are different kinds and degrees of sameness. The main problem is then to point out a “sufficient degree of likeness or sameness”.  The simplest form of inference is making use of a pattern, proxy, example or sample. If the sample “exactly represents the texture, appearance, and general nature” of a certain commodity, then what is true for the sample will also be true for the commodity as a whole. This approach evades defining similarity, as it presupposes that the sample is an ‘exact representation’ of the commodity whereas it is unclear under what conditions that would be the case. All processes of inference are based on the principle of substitution. All knowledge is derived from sensual experience, which implies that all knowledge is inductive.  Deduction is the inverse process of induction, and both rely upon the nature of identity. 