[48] To which De Morgan drew attention in a letter; see (Mrs.) S. E. De Morgan, Memoir of Augustus De Morgan, London, 1882, p. 324.
[49] Pa. Ma., p. 16.
[50] However, here, for the printer’s convenience, we depart from Mr. Russell’s usage so far as to write “not” for a curly minus sign.
CHAPTER XIX
IMPLICATION
A good illustration of the fact that what is called “implication” in logic is such that a false proposition implies any other proposition, true or false, is given by Lewis Carroll’s puzzle of the three barbers.[51]
Allen, Brown, and Carr keep a barber’s shop together; so that one of them must be in during working hours. Allen has lately had an illness of such a nature that, if Allen is out, Brown must be accompanying him. Further, if Carr is out, then, if Allen is out, Brown must be in for obvious business reasons. The problem is, may Carr ever go out?