In the last lesson, I introduced an argument form called Modus
Ponens.  The importance of knowing about it is that any argument that bears
its form is valid.  In this lesson, I will introduce argument forms for other
valid arguments.  These forms are referred to together as the rules of
inference.  So that the set is complete I will also include Modus Ponens.

        Modus Ponens (M.P.)             Modus Tollens (M.T.)
        P -> Q                          P -> Q
        P                               ~Q
        ------                          ------
        Q                               ~P

        Hypothtical Syllogism (H.S.)    Disjunctive Syllogism (D.S.)
        P -> Q                          P v Q
        Q -> R                          ~P
        ------                          ------
        P -> R                          Q

        Constructive Dilemma (C.D.)     Destructive Dilemma (D.D.)
        (P -> Q) & (R -> S)             (P -> Q) & (R -> S)
        P v R                           ~Q v ~S
        -------------------             -------------------
        Q v S                           ~P v ~S

        Simplification (Simp.)          Conjunction (Conj.)
        P & Q                           P
        -----                           Q
        P                               -----
                                        P & Q

        Addition
        P
        -----
        P v Q