We like putting things in pairs. Black, white; good, evil; you, me. Always binary. Even the computer you're using now is based on this; everything is "on" or "off".
A more detailed example: people seem to like two-party systems. Here in Britain, we have the Labour government and the Conservative opposition. Even the system of government versus opposition encourages there being two significant parties. Third parties get written off, regardless of their actual importance. Look at the Liberal Democrats, and in Scotland, the SNP. Both have, in many areas, out-performed one or both of the two main parties, and yet lose thousands of vote because they are seen as also-rans. It's a similar situation in the USA, with the Republicans and Democrats. Few take Ralph Nader or the Libertarians to be contenders.
Sure, some things are put on a sliding scale. Punching someone is less evil than murder. Black is darker than dark grey. Given the slightest opportunity, though, we'll reduce it right back down to two. A thug who punches another person is bad, so is a murderer. End of story. We'll feel virtuous for choosing salad over pizza, just as we will for giving a few pounds to Oxfam and saving maybe five people's lives. The question of degree is glossed over. Similarly, give us four things, and we'll promptly split them into two pairs. North and south; east and west.
So why is this? Why two?
Unless a psychology graduate, or an anthropology professor, wants to correct me, I'll have to say: we don't know. We can speculate, though.
I've been giving this a fair amount of thought recently. After drawing several blanks, I started looking at things that come in pairs. What is there? Up and down. Well, they're our concepts. Just our way of looking at things. What else? Eyes. Arms. Legs. Thumbs. See a pattern?
Look at the left half of your body. Someone else's, if you prefer. Maybe look at George W. Bush. Note the features. Now look at the right. Give or take the odd heart, the occasional squint or scar, they're mirror images.
We're symmetrical. No great revelation there. Two sets of ribs, two halves of the brain. Could this have something to do with it? According to various research projects, several animals think like we do, albeit not at the same level: Washoe the chimp and Griffin the parrot come to mind. They can grasp concepts like "good" and "bad"; pretty much any creature can decide whether it likes a sensation or not. Could this be related to the way we come in two halves?
If it is, it poses some interesting questions. Perhaps this is why three, and seven, are common numbers in mysticism all over the world - we can't reduce them down to two easily, so they're strange to us. Take starfish - a variety with five arms, for example. It's possible that they'd have a completely different way of looking at things, based on fives. An extraterrestrial species with three-way symmetry, in the unlikely event that we should have contact with one, might think in threes. Such a basis for thought would be, in the truest sense of the word, alien to us.
Of course, this is all idle speculation. I'm not a biologist; I've never studied philosophy or psychology. One thing I can do is reduce one final thing to two before I sign off: this hypothesis is either valid, or it isn't.
Owen is off in search of the second dimension