 It is worth noticing that the growing sophistication of ideas about how theoretical geometry relates to practical experience, and about the nature of the knowledge that geometry supplies, belong to a family of changes across all of mathematics by 1900. An autonomous discipline of mathematics emerged that placed an increasing emphasis on formal aspects of the subject and offered a complicated and often distant relationship with the world of experience. This modernist turn in mathematics is discussed in various places (see Gray 2008 and the literature cited there).