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If a person stopped thinking, they would no longer be human.
Literal
By-chance thinking thing [object-を] stop-[conditional-ば], that person [topic-は] anymore-もはや human [is-not-ではなくなる] [probably-であろう].
A philosophical aphorism. もはや ('any longer, anymore') is a formal/literary negative-polarity adverb — specifically for something that 'is no longer' the case. ではなくなる ('to cease being X') combines ではない (negation) with ~くなる (inchoative) — 'comes to not be.' であろう is the formal written/classical variant of だろう.