Sheffer’s work. The manner in which other truth-functions are constructed out of “not- p and not- q ” is easy to see. “Not- p and not- p ” is equivalent to “not- p ”, hence we obtain a definition of negation in terms of our primitive function: hence we can define “ p or q ”, since this is the negation of “not- p and not- q ”, i.e. of our primitive function. The development of other truth-functions out of “not-