Full Idea
Leibniz seems to be the first modern philosopher to focus on sentence-sized units that he called propositions. The Empiricists among the moderns focused on word-sized units like ideas.
Gist of Idea
Leibniz was the first modern to focus on sentence-sized units (where empiricists preferred word-size)
Source
report of Gottfried Leibniz (works [1690]) by William D. Hart - The Evolution of Logic 2
Book Reference
Hart,W.D.: 'The Evolution of Logic' [CUP 2010], p.32
A Reaction
Historically, the sentential logic of the Stoics has a claim to have started this one. I find my initial sympathies to be with the empiricists.
Related Idea
Idea 20787 A proposition is what can be asserted or denied on its own [Chrysippus]