"What set Napier to work on creating tables which were to enable multiplication to be performed by a process of addition? What first gave him the idea of any such thing? ...there is a peculiarity in the form of his investigations which gives us a useful clue. He usually frames his propositions as though they applied exclusively or at all events specially to sines. Now it is evident that all that concerns logarithms must relate to numbers generally, and that their being sines has no bearing on the matter. Hence his confining his work to sines must indicate that he set out with the idea of working on them only, and that it was only at a later stage and perhaps incidentally that he realised that his results could with like advantage be applied to numbers generally. I conclude from this that his original idea was only to construct tables that would enable the product of two sines to be readily ascertained. If I am right in this, the suggestion may well have come to him from his familiarity with the well known trigonometrical formula:— \sin A \sin B = \frac{1}{2} ({\cos(A - B) - \cos(A + B)})"