  In this short letter we review Schwinger's formulation of Quantum Mechanics
and we argue that the mathematical structure behind Schwinger's "Symbolism of
Atomic Measurements" is that of a groupoid. In this framework, both the Hilbert
space (Schr\"{o}dinger picture) and the $C^{*}$-algebra (Heisenberg picture) of
the system turn out to be derived concepts, that is, they arise from the
underlying groupoid structure.
