Godel 1930 / Malcev 1936: a set of first-order sentences has a model iff every finite subset has a model; foundation of model theory + non-standard analysis (Robinson 1961).
Godel 1930 / Malcev 1936: a set of first-order sentences has a model iff every finite subset has a model; foundation of model theory + non-standard analysis (Robinson 1961).