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 and fifth senses identified by the Oxford English Dictionary: 4. Power actively and efficiently displayed or exerted. 5. Power not necessarily manifested in action; ability or capacity to produce an effect. Modern subatomic physics, of course, recognises no such distinction between 'being' and 'doing'; at a fundamental level, matter-as-substance and matter-as-energy are interchangeable (and, as I shall argue towards the end of the study, Diderot himself comes close to a similar position). Nevertheless, the division is both justifiable and useful within the context of eighteenth-century philosophies of nature. For, as many scholars have pointed out, the trend towards nature as an integrated, active phenomenon, in place of the cartesian view of passive étendue only incidentally endowed with motion, was crucial to the development of scientific thought in the mid-eighteenth century. Debate and development on such issues as Newtonian attraction, inertia, electricity and magnetism, chemical reactions, not only contributed directly to the advancement of physics and chemistry, but also (like cartesian mechanism) impinged upon the perennial biological questions, themselves being investigated from a new and exciting angle. As a philosopher rather than a practising scientist, Diderot was ideally placed to draw freely and creatively on all these areas, and his speculations on what we might call 'the nature of nature' are highly characteristic of the new approach. He comes increasingly to discuss and define natural phenomena (organic and inorganic alike) from the point of view of nature's powers - in the spirit of Renaissance naturalism, but from the perspective of up-to-date scientific findings. It is in this sense that I refer to a 'concept of physical energy'. Given the organic quality of Diderot's thought, it is not surprising to find the idea of energy recurring in other areas of his works. If man is composed of matter - active matter - than all human activity, be it moral, political, aesthetic, becomes capable of interpretation in terms of energy. I share Chouillet's conviction that this is a crucial aspect of Diderot's overall philosophy, which deserves to be more widely recognised and more fully understood.