"We teach our students that prices are set by supply and demand. But this logic is exactly backwards. Supply and demand are concepts economists invented to help us understand prices. The emergence of prices from the tendency of human beings to truck, barter and exchange caused the invention of supply and demand as a way for our brains to organize our thinking about the orderliness of prices and the behavior such order makes possible."
This is worth some thought. Check out the essay he links to as well.