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When unreason prevails, reason retreats.
Literal
Unreason [subject-が] pass [conditional-ば] reason [subject-が] retreats.
A classic Japanese ことわざ observing social dynamics: when injustice is accepted, justice recedes. The two halves balance — 無理が通れば (if unreason prevails) / 道理が引っ込む (reason retreats). 通る ('to pass, to get through') + 引っ込む ('to retreat, to draw back'). A pithy observation.