We present a simple explanation of the physical origin of the Born rule.
We prove that any physical theory that assigns probabilities to the results of ideal
measurements must assign probabilities satisfying and saturating the Born rule if
nothing (e.g., constraints or physical laws) restricts these results. The proof uses
the graph-theoretical approach to general probabilistic theories and ideal
measurements are operationally defined through necessary conditions. This
result can be seen as a proof of Max Born’s 1926 conjecture that, the agreement
between quantum theory and experiment is a “pre-established harmony founded
on the nonexistence of conditions for a causal evolution”.