Arbitrary means based on personal choice, chance, or random decision rather than on clear rules, reasons, or systems.
From Latin "arbitrarius" meaning "depending on the will of an arbiter," from "arbiter" (judge). It shifted from “decided by a judge” to “decided by someone’s will, not by principle.”
So many rules we treat as natural—like which side of the road to drive on—are actually arbitrary agreements. Once you see that, you realize how much of life is held together by shared habits, not unbreakable laws.
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