The present chapteraims to show some correlations between Charles Peirce's life, his intellectual habits as a mathematician, his semiotic theory, and his practice as a geodesist. For this purpose, we make use of Peirce's ideas about the nature of visual experience, some facts of his intellectual biography, and his definitions of sign and the term "virtual". The principal thesis of the chapter is that Peirce's mature pragmatist and semiotic ideas find some support in his early practice as a scientist and a mathematician, thus providing an interesting example of the intersection of scientific practice and philosophical speculation.