Engels on the limitations of formal logic

Engels, satirizing the laws of identity and contradiction in formal logic, wrote, “To the metaphysician, things and their mental reflexes, ideas, are isolated, are to be considered one after the other and apart from each other, are objects of investigation fixed, rigid, given once and for all. He thinks in absolutely irreconcilable antitheses. ‘His communication is yea, yea; nay, nay; for whatsoever is more than these cometh of evil.’ For him a thing either exists or does not exist; a thing can not at the same time be itself and something else. Positive and negative absolutely exclude one another; cause and effect stand in a rigid antithesis one to the other.” Anti-Dühring (Moscow: Progress Publishers, 1969), 31.