The conclusion follows from the premises, without any further assumptions that might turn out to be false.
Any risk of error lies entirely with the premises, as opposed to the reasoning.
Do sentences exhibit grammatical structures that are not obvious?
And if the logical structure of a thought can diverge from the grammatical structure of a sentence that is used to express the thought, how should we construe proposals about the logical forms of inferences like (1-6)?
A given speaker might use ‘I am tired’ to express a false proposition, while another speaker uses the same sentence at the same time to express a true proposition.
What counts as being tired can also vary across conversations.
By contrast, examples like (4–6) illustrate reasoning that involves at least some risk of going wrong—from correct premises to a mistaken conclusion. John might dance whenever Mary sings, but also sometimes when Mary doesn't sing.
Similarly, with regard to (5), Tweety might turn out to be a bird that cannot fly.
This leaves it open what propositions are: sentences, statements, states of affairs, or whatever.
But let's assume that declarative sentences can be used to express propositions.