  Propositions exist, for Schiffer, but unlike rocks or cats, there is nothing more to them than what our concept of a proposition guarantees. One may call them “abstract entities,” if one likes, but this label should not encourage the thought that our minds can reach beyond the physical world to make contact with denizens of a Platonic universe. We know about propositions, not by interacting with them, as we do with rocks and cats, but by being participants in certain sorts of linguistic or conceptual practice. It’s because we speak or think in certain ways that we are able to know about propositions.