A field of terms reaches syntactic capacity when its distinctions can combine without collapsing and begin to produce propositions previously unavailable. Vocabulary alone is insufficient. Thousands of isolated concepts may form an archive without forming a language. ArgumentSeal names the moment when lexical accumulation acquires grammatical force: terms begin to constrain, provoke and transform one another, producing arguments that cannot be reduced to any single component. Semantic friction between heterogeneous roots creates the interval in which new meaning appears. A useful proposition then stabilises long enough to support further thought while remaining visible, revisable and open to recombination. The seal is therefore temporary rather than absolute; it marks the point at which an argument has achieved sufficient internal pressure to travel. This transforms conceptual invention from naming into composition. The measure of a vocabulary is not how many terms it contains but what relations it permits, what distinctions it preserves under combination and what sentences the world could not produce before them. Grammar emerges from disciplined density rather than accumulation.