Simplification and generalization is a survival instinct. To be honest, I can't remember whether I picked up that tidbit in a psych class or out of a fantasy novel, but as an opinion I think it's pretty straightforward and accurate.
I disagree that duality has anything to do with right-and-left brains. There are a whole bunch of numbers out there that are just as popular (in some cultures more than other) than two. Three, for instance. Maiden/mother/crone, father/mother/child, grandparent/parent/grandchild, earth/sea/sky (which also extends into many nations' armed forces), morning/afternoon/night, past/present/future, body/mind/spirit, underworld/world/overworld, the triangle...three is just as powerful a number.
There's also "nothing"--nirvana. "Four"--fire/water/earth/air, north/south/east/west, left-right/back/forward, the four-pointed star, the cross, the fyflot (aka swastika), black/white/red/yellow (the native american colours), and the four seasons. "Five"--earth/air/sky/fire/spirit, or the same thing except 'wood' instead of spirit in the Orient.
"Two" is just one of a number of sacred numbers, with all the examples you presented and more. However, I think part of its power is in its description of opposites, which brings it into the 'simplification' thing it took me so long to tie into. "Us and them" is pretty much the most simple and the most primal set of opposites, which is the chief argument for 'simple is a survival instinct'. When opposites are presented as a choice, the better choice is obviously the one best suited for survival. "Food or hunger", "warmth or freezing", "fight or flight"...I think the duality you're talking about is because our need for simplification makes 'either/or' choices the easiest, and 'easiest' generally means a greater chance for survival. I don't think it has to do with left and right brains, if for no other reason than because we're certainly capable of decisions with more than two options.
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I disagree that duality has anything to do with right-and-left brains. There are a whole bunch of numbers out there that are just as popular (in some cultures more than other) than two. Three, for instance. Maiden/mother/crone, father/mother/child, grandparent/parent/grandchild, earth/sea/sky (which also extends into many nations' armed forces), morning/afternoon/night, past/present/future, body/mind/spirit, underworld/world/overworld, the triangle...three is just as powerful a number.
There's also "nothing"--nirvana. "Four"--fire/water/earth/air, north/south/east/west, left-right/back/forward, the four-pointed star, the cross, the fyflot (aka swastika), black/white/red/yellow (the native american colours), and the four seasons. "Five"--earth/air/sky/fire/spirit, or the same thing except 'wood' instead of spirit in the Orient.
"Two" is just one of a number of sacred numbers, with all the examples you presented and more. However, I think part of its power is in its description of opposites, which brings it into the 'simplification' thing it took me so long to tie into. "Us and them" is pretty much the most simple and the most primal set of opposites, which is the chief argument for 'simple is a survival instinct'. When opposites are presented as a choice, the better choice is obviously the one best suited for survival. "Food or hunger", "warmth or freezing", "fight or flight"...I think the duality you're talking about is because our need for simplification makes 'either/or' choices the easiest, and 'easiest' generally means a greater chance for survival. I don't think it has to do with left and right brains, if for no other reason than because we're certainly capable of decisions with more than two options.