Dee promoted the sciences of navigation and cartography. He studied closely with Gerardus Mercator and owned an important collection of maps, globes and astronomical instruments. He developed new instruments and special navigational techniques for use in polar regions. Dee served as an advisor to English voyages of discovery, and personally selected pilots and trained them in navigation. He believed that mathematics (which he understood mystically) was central to human learning. The centrality of mathematics to Dee's vision makes him to that extent more modern than Francis Bacon, though some scholars believe Bacon purposely downplayed mathematics in the anti-occult atmosphere of the reign of James I. Although Dee's understanding of the role of mathematics differs much from ours, its promotion outside the universities was an enduring achievement. For most of his writings, Dee chose English, rather than Latin, to make them accessible to the public. His "Mathematical Preface" to Euclid was meant to promote the study and application of mathematics by those without a university education, and was popular and influential among the "mechanicians": a growing class of technical craftsmen and artisans. Dee's preface includes demonstrations of mathematical principles that readers could perform themselves without special education or training.