Jacob de Gelder and the mathematic ideology in The Netherlands (1800-1840) During the period which is commonly called the Enlightenment, changes occurred in the social perception of mathematics. The teaching of mathematics used to be almost strictly academic and was considered to be an important part of knowledge to those who had a scientific career in view. In The Netherlands, during the first part of the nineteenth century, people began to think that mathematics was indispensable to everyone: even to the craftsmen who until then were educated within the guilds on a purely practical basis. Because of their character these views will here be called the 'mathematical ideology'. In The Netherlands, the reception of this mathematical ideology may be illustrated by the biography of the Dutch mathematician Jacob de Gelder (1765-1848). This man started off as an unimportant schoolteacher during the late eighties of the eighteenth century. He took an interest in mathematics and educated himself and his pupils in this science. In his teaching as well as in his writings he would proclaim the mathematical ideology. He made a remarkable career and was appointed professor of mathematics at Leiden University in 1819. The following three aspects constituted De Gelder's views: 1. Mathematics became more and more the basis of reasoning. The argument was even reversed: every good argument was called a mathematical one and if an argument was considered not convincingly enough, a mathematical formula was added, a phenomenon easily illustrated by several of De Gelder's manuscripts. 2. Mathematics became a necessary prerequisite for the study of all other sciences. Therefore, it actually became a standard part of the curriculum of all Dutch schools preparing students for the university. 3. Mathematics even became a necessity for craftsmen. De Gelder was convinced, and the Government agreed with him, that the teaching of mathematics to labourers would result in a better and faster development of industry in The Netherlands, which in ...