One can use sentences to present arguments, some of which are valid. Sentences are complex linguistic expressions that exhibit grammatical structure. And the grammatical properties of sentences need not be obvious. As discussed in this entry, certain arguments seem to be valid because the relevant premises and conclusions exhibit nonobvious logical structure. But this raises questions about what logical structure is and how it is related to grammatical structure.

Patterns of Reasoning

An ancient thought is that premises and conclusions have parts and that valid arguments exhibit valid forms, like the following: Q if P, and P; so Q. One can say that the variables (in bold) range over propositions, leaving it open for now what propositions are: sentences of some (perhaps unspoken) language, abstract states of affairs, or whatever. One can also assume that declarative sentences can be used, in contexts, to indicate or express...