The title of this work may seem to beg an important question, since it rests on the assumption that Diderot has a ‘concept of physical energy’. Indeed the aim of the study is, in part, to assemble evidence in support of the acte de foi implicit in its title. I am using ‘physical energy’ in a loose sense, as a convenient term to denote ‘what matter can do’ as distinct from ‘what matter is made of’. Hence it may be taken as broadly synonymous with ‘power’ or ‘force’, encompassing both active and potential forms, and thus corresponding to a combination of the fourth...