Under the 'New Math' initiative, conceptual abstraction gained the central role in mathematics education. Students received lessons in set theory, which is what mathematicians actually use to construct the set of real numbers, something advanced undergraduates learned in a course on real analysis. Arithmetic with bases other than ten was also taught. However, this educational initiative faced strong opposition, not just from teachers, many of whom struggled to understand the new material, let alone teach it, but also parents, who had problems helping their children with homework. It was criticized by experts, too. In a 1965 essay, physicist Richard Feynman argued, "first there must be freedom of thought; second, we do not want to teach just words; and third, subjects should not be introduced without explaining the purpose or reason, or without giving any way in which the material could be really used to discover something interesting. I don't think it is worthwhile teaching such material." In his 1973 book, "", mathematician and historian of mathematics Morris Kline observed that it was "practically impossible" to learn new mathematical creations without first understanding the old ones, and that "abstraction is not the first stage, but the last stage, in a mathematical development." Kline criticized the authors of the 'New Math' textbooks, not for their mathematical faculty, but rather their narrow approach to mathematics, and their limited understanding of pedagogy and educational psychology. Mathematician George F. Simmons wrote in the algebra section of his book "Precalculus Mathematics in a Nutshell" (1981) that the New Math produced students who had "heard of the commutative law, but did not know the multiplication table."