The word `calculus' usually conjures up images of integrals and differential
equations. But the word more generally
means `a method of reasoning by the manipulation of symbols' and it is
this more general sense that I am using. The word has an interesting origin:
it is Latin meaning a small pebble. The medical term for a kidney or gall
stone is calculus. In medieval Europe, store keepers used pebbles
(symbols) manipulated on a `counting board' to add up the cost of goods
- much like a cribbage board is used today. The pebbles have been replaced
by cash registers, but board is still there, it's name shortened to `counter'.
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