Full Idea
Consistency is a modal notion: a set of propositions is consistent iff all the members of the set could be true together.
Clarification
'Could' is a modal notion, depending on possibility rather than actuality
Gist of Idea
Consistency is modal, saying propositions are consistent if they could be true together
Source
Joseph Melia (Modality [2003], Ch.6)
Book Reference
Melia,Joseph: 'Modality' [Acumen 2003], p.129
A Reaction
This shows why Kantian ethics, for example, needs a metaphysical underpinning. Maybe Kant should have believed in the reality of Leibnizian possible worlds? An account of reason requires an account of necessity and possibility.