I have noticed a significant decline over the years in the ability of my students to estimate quantities, and have attributed it to an increased reliance on calculators and computers, but it may be a consequence of more subtle differences in brain development. In this article, Jim Holt describes the research of Stanislas Dehaene, a young Frenchman who is trying to find the loci of arithmetic ability in the brain. Using studies across cultures and species, with heavy reliance on high-resolution brain scans, he has evidence that different number abilities are carried out in three different parts of the brain; "number sense" is evolved and is found in other species, "numerals" and "number words" are linked to culture. Chinese four year olds can usually count to 40 (in Chinese, of course) while English speakers of the same age can make it only to about 15. Dehaene thinks that Piaget seriously underestimated the arithmetic abilities of young children, and he doesn't think much of "New Math" approaches to teaching. Dehaene's research is more pertinent to the learning of arithmetic than higher mathematics, but perhaps this approach will disclose why my students aren't much good at calculus (after passing three semesters worth of courses).