The so-called ‘crank’ may be quite original in his ideas. … Invention, however, in the engineering sense involves originality; but not that alone, if the results are to be of value. There is imagination more or less fertile, but with it a knowledge of what has been done before, carried perhaps by the memory, together with a sense of the present or prospective needs in art or industry. Necessity is not always the mother of invention. It may be prevision.