 These schematic formulations employ variables, indicated in bold. Following a long tradition, let’s use the word ‘proposition’ as a term of art for whatever these variables range over.  Propositions  are potential premises/conclusions, which can be endorsed or rejected. So they are, presumably, things that can be evaluated for truth or falsity. This leaves room for various proposals about what propositions are: sentences, statements, states of affairs, etc. But let’s assume that declarative sentences can be used to express propositions; see, e.g., Cartwright (1962) and the essay on  structured propositions.