。
Exceptions prove the rule.
Literal
Exceptions [topic-は] principle [of-の] existence [of-の] proof.
A direct Japanese rendering of the English proverb. 例外 ('exception') + 原則 ('principle, rule') + ある ('exists') + 証拠 ('proof') — literally 'exceptions are proof that a rule exists.' Copula dropped at the end, proverb-style.