The discussion of the nature of atoms which grew out of Poysson's problem suggested to Gassendi that physical atomism would be an indispensable key to the future development of physics. However, this relatively brief scholarly exchange was one among many similar exchanges in which Gassendi had observed that incompatible definitions of concepts and incommensurable rival solutions to the problems at hand had rendered the exchanges incoherent. The ambiguities which characterized Poysson's problem were also endemic to the interpretation of several major scientific investigations in astronomy and optics which Gassendi undertook during the 1630s and 1640s. Even when these investigations were based on a specific set of observations or experiments, at least four sorts of disagreements usually arose in the subsequent scholarly exchange. (1) Different observers could not always agree about what exactly they saw when they observed celestial phenomena through a telescope or when they made naked-eye observations, for example, of the apparent magnitude of the sun at different elevations above the horizon. (2) Their explanations of what they saw were often expressed in terms of widely divergent optical theories about the constitution and behavior of light.