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Justice is expressed through actions.
Literal
Justice [subject-が] action [by-means-of-によって] is-expressed.
A formal, aphoristic statement using the passive 表される. The particle によって marks the agent or means in a passive construction — here 'by actions.' The passive form 表される is from 表す (to express/represent), making the sentence read as 'justice is expressed by actions.' This is a written-register proverb-like declaration; the passive gives it a detached, universal quality typical of philosophical maxims.