B. Butterworth, author of the new book The Mathematical Brain, proposes that your brain boasts a tiny module of cells -- just over your left ear -- that endows
you with a sense of number. These cells allow you, for example, to grasp instantly "fourness," say the number of corners on a square without counting them
one-by-one. Unfortunately, this capability usually does not exceed fiveness. If there were 10 people on a corner, you would have to count them individually --
if you are normal.

But some people are abnormal. Recall from SF#125, the savant who could tell at a glance that 111 matches littered the floor without counting each
individually. He grasped 111-ness! At the other end of the scale, Signora Gaddi cannot even distinguish that 20 is greater then 10. She cannot use the
telephone or catch numbered busses. Facts involving numbers above four are a mystery to her. Even when there are four or fewer objects, she must count them
one-byone. Nevertheless, Gaddi's intelligence and social skills are normal. She lost her number-savvy when she suffered a stroke that apparently short-circuited that number module over her ear.

Are other mammals equipped with number modules? No one knows. And what forces encouraged the human brain to sprout a few extra cells on the inferior
parietal lobule; that is, the number module? Did the sense of fiveness give some mutant ancient humans superiority over less-evolved humans? Finally, what
factors pushed the number module's capacity to 111 in that savant, or is the savant's talent intrinsic to all of us but somehow suppressed?

(Dahaene, Stanislas; "Counting on Our Brains," Nature, 401:114, 1999. Motluk, Alison; "True Grit," New Scientist, p. 46, July 3, 1999.)