In ‘The Critic of Arguments’ (1892), Peirce adopts a notion that is even closer to that of a propositional function. There he develops the concept of the ‘rhema’. He says the rhema is like a relative term, but it is not a term. It contains a copula, that is, when joined to the correct number of arguments it produces an assertion. For example, ‘__ is bought by __ from __ for __’ is a four-place rhema. Applying it to four objects a, b, c, and d produces the assertion that  a is bought by b from c for d (ibid. 420). 