 It may, however, be objected that it is problematic to operate with two different kinds of propositions: 1) propositional constants with truth-values that do not vary with the chronicles and 2) other wffs with truth-values that may vary with the chronicles. Thomason (1970, p. 280) has pointed out that this distinction means that substitution of propositions in the system will have to be restricted, since we will not be allowed to substitute a propositional constant with an arbitrary wff. Prior was aware of this, but he argued that it is in fact possible to handle a system with restrictions on the substitution-rules. (See Prior 1967, p. 122 ff.)