The "Free Discussion" between Richard Price and Joseph Priestley (1778) originated as a correspondence between the two after the publication of Priestley's "Disquisitions on Matter and Spirit", his most important philosophical work (1777). At the time it was thought remarkable that a controversey such as this could be conducted so amicably, but then the two were close friends. Nevertheless their philosophical, as opposed to their oft mentioned political views, were at opposite ends of a spectrum. Price believed that matter was passive and that explanations of such concepts as force and action at a distance were best expressed in mathematical terms. In this he was following a long tradition of British Mathematical Scholarship derived from Colin Maclaurin. Priestley, following a different tradition derived from Rowning and Boscovitch, thought that matter was not merely to be acted on, as Price would have argued, but was itself, and of itself, active. This led to Priestley denying the conventional dualism of matter and spirit. The result is that this correspondence is perhaps the clearest expression of the differences between materialism and immaterialism in 18th century philosophy.