Characters of second intention are characters which are brought to our knowledge, not by observation of their subjects, but by observation of logical forms. Relatives of second intention are of high importance in logic, as might be anticipated. Especially so are those which express the numbers of collections. All such arithmetical relatives are expressible in terms of three fundamental arithmetical relatives, a monad, a dyad, and a triad.

Without these the most elementary requisites of logic cannot be fulfilled.