This article perked up my neurons.
http://opinionator.blogs.nytimes.com/2010/02/14/the-enemy-of-my-enemy/?pagemode=print
The Enemy of My Enemy
Subtraction can generate negative numbers. If I try to take 6 cookies away from you but you only have 2, I can’t do it — except in my mind, where you now have negative 4 cookies, whatever that means.
Subtraction forces us to expand our conception of what numbers are. Negative numbers are a lot more abstract than positive numbers — you can’t see negative 4 cookies and certainly can’t eat them — but you can think about them, and you have to, in all aspects of daily life, from debts and overdrafts to contending with freezing temperatures and parking garages.
Still, many of us haven’t quite made peace with negative numbers.
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