 Now say that a term \(\tau\) occurring in a formula \(\varphi\) is free in \(\varphi\) if \(\tau\) is either an individual constant or a variable with a free occurrence in \(\varphi,\) and say that \(\varphi\) is singular if some term is free in it and wholly general otherwise. Given this, Prior axiomatizes the left-to-right direction of principle S as follows: