The proof begins with a small volume element and compares the net flux through its faces with the divergence inside that element. As the element becomes infinitesimal, this local comparison provides the basic relation needed for the full argument. Repeating that relation throughout a partition connects field behavior at individual locations with the flux measured over the entire boundary.
Each internal face belongs to two neighboring volume elements, but the elements assign opposite outward directions to that same face. Consequently, the flux leaving one element matches the flux entering the adjacent element with the opposite sign. When all elemental fluxes are added, these internal contributions cancel pairwise, so only faces on the external boundary remain.
The argument relies on a vector field that is sufficiently smooth within the region and on a bounded volume whose boundary can be treated as the enclosing surface. Smoothness supports the local divergence calculation and the passage to infinitesimal elements, while boundedness ensures that the partition has a finite outer boundary where the remaining flux is evaluated.
First partition the bounded volume into many small elements. Next apply the local flux-divergence relation to each element and add the resulting expressions. Shared internal faces cancel because adjacent elements use opposite orientations. Finally refine the partition so the elements become infinitesimal, leaving the total flux through the outer boundary equal to the accumulated volume contribution.
Divergence describes the net outward behavior of a field at a local point, whereas boundary flux records the combined effect over an enclosing surface. The proof shows why integrating the local quantity throughout a volume reproduces that global measurement. This connection lets physicists interpret surface observations through the field behavior distributed inside the region.
The theorem provides a mathematical basis for analyzing conservation laws, fluid flow, electrostatics, and heat transfer. In each setting, a volume description involving divergence can be related to a flux measured across the surrounding surface. Its value is therefore both computational and interpretive: it helps translate between behavior inside a region and transport across its boundary.