Signed Measures as the Linear Envelope of Positive Measures

Signed measures are traditionally introduced as countably additive set functions that may take both positive and negative values. The classical Jordan decomposition theorem shows that every finite signed measure can be expressed uniquely as the difference of two mutually singular positive measures. While this theorem provides a structural description of signed measures, it does not characterize them by a universal property.
We show that, for every measurable space, the abelian group of finite signed measures satisfies a universal property with respect to the commutative monoid of finite positive measures: every additive map from positive measures into an abelian group extends uniquely to a group homomorphism on signed measures. In this sense, signed measures are the canonical additive extension of positive measure theory.
We compare this characterization with classical Grothendieck completion, clarifying both the analogy and the additional structure arising from countable additivity and Jordan decomposition. This places signed measures within the familiar framework of additive completion and linearization, providing a conceptual explanation for their role in analysis and probability.