ABSTRACT
In 1910, in the first volume of Principia Mathematica, Whitehead and Russell presented an axiomatization of PC, with disjunction and negation as primitive operators. In order to avoid unnecessary bracketing one adopt at once with respect to PM the conventions used hitherto, allowing us to drop outside brackets and introduce or drop those pairs of which the left-hand member lies between negations with the same scope. It should be clear from considerations that the axioms are the ultimate premisses in every proof, it is both sound in theory and convenient in practice to use as a premiss any established thesis, whether it be an axiom or a previously proved theorem.
