# Pieri’s *Point and Sphere* Memoir

## Abstract

This chapter contains an English translation of Pieri’s 1908a memoir, *Elementary Geometry Based on the Notions of Point and Sphere*.^{1} The work had two main goals. First, it presented elementary Euclidean geometry as a hypothetical-deductive system, and showed that all its notions and postulates can be defined and formulated in terms of the notion *point* and the relation that holds between points *a,b,c* just when *a,b* are *equidistant* from *c*. As noted in section 5.2, this result gave rise, over decades, to a stream of related research that still continues. The paper’s title reflects Pieri’s extensive use of elementary set theory in developing geometry from his postulates: he defined the sphere through *b* centered at *c* as the set of all points *a* such that *a* and *b* are equidistant from *c*. Pieri’s second aim was to foster more extensive use of properties of spheres in presenting elementary geometry, even in school courses. In this regard, he seems to have had less impact, even though this memoir presents many useful examples. A third aim, which Pieri had already pursued for a decade, was to promote the use of transformations in elementary geometry. Pieri introduced various geometric transformations early through definitions, and employed them extensively throughout the paper, following paths already explored in his 1900a *Point and Motion* memoir. Finally, Pieri followed the strategy of *fusionism* in developing plane and solid geometry together.^{2}

## Keywords

Elementary Geometry Arbitrary Rotation Polar Sphere Convex Angle Noncollinear Point## Preview

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