Duality is the simplest form of intelligence. Primitive intelligence. If intelligence is the ability to differentiate and recognize divergent patterns, a duality represents the first step in this process only--differentiating between two juvenile elements, the positive and the negative--but going no further than that. Instead of progressing and evolving into an ever-increasing, multilayered complexity, the "simple mind" stops at just two notions:
Black and white. Good and evil. Light and darkness. Being and nothingness. Body and soul. Reality and fantasy. Pleasure and suffering. Courage and cowardice. Flesh and spirit. Love and hate. Relative and absolute. Oppression and freedom. Emotion and logic. Motion and stillness. Action and apathy. Mind and matter. Heaven and earth. Yin and yang. Civilized and savage. Sanity and insanity. Fact and belief. Truth and falsehood. Art and absurdity. Language and gibberish. Desire and necessity. Accident and design. Volition and indifference. Altruism and selfishness. Purity and impurity. Damnation and liberation. Paradise and hell. Angel and demon. Beauty and ugliness. Conflict and peace. Life and death. Fear and trust. Virgin and whore. Strong and weak. Smart and stupid. Reason and irrationality. Order and chaos. Greatness and smallness. Superiority and inferiority. Young and old. Crime and law. Master and servant. Right and wrong. Vice and virtue. Wickedness morality. Saint and sinner. Creation and destruction. Grace and abomination. God and the Devil. The real and the ideal.
Any concept which is sustained solely by a duality, "black and white thinking" (also known in psychology as "splitting"), is a primitive, ignorant, and unintelligent viewpoint. A self-affirming delusion. The product of an under-achieving intellect.
The universe is a known plurality: The manifestation of not just many grays, but also many blues, and reds, and greens. Think about it the next time you start to make an arbitrary comparison ... Is it valid or is it a false dichotomy? Or is it a validly false dichotomy? Or an invalidity true dichotomy? Or a conditionally neutral, split variable of semantically unlimited resolutions too diverse to enumerate?
So when someone says we need opposites in order to understand differences, this is a fundamental fallacy in thought that is reliant on a pre-existing cognitive bias. The psychological need to frame everything in dualistic terms. This need is not logical, but is likely genetic or learned behavior. It can therefore be overcome with practice.