William Cullen, lecturer in chemistry at Glasgow and Edinburgh Universities, spent many years formulating his own theory of heat and combustion, the most developed version of which appears in a little-known set of lecture notes of 1765. Cullen's theory is of particular interest to historians of chemistry as an example of his ideal of ‘philosophical chemistry’, an autonomous branch of natural philosophy distinct from the mechanical philosophy, with its own general laws and explanations of phenomena justified by observation. The theory assimilated Joseph Black's recent discovery of fixed air as well as Cullen's investigations of the generation of heat in chemical operations. It was formulated just one year before British chemists' sudden identification of new ‘airs’ was dramatically to change the field of phlogiston theory. The theory differs in important ways from any version yet discussed. It successfully brought both heat and elective attraction within its explanatory domain. It set out a causal hierarchy which reversed the usual pattern evinced in earlier sets of lecture notes, subordinating the mechanical to the chemical in the form of Cullen's theory of elective attraction. The paper argues that Cullen was attempting to bring the study of heat as well as combustion within the bounds of his ‘philosophical chemistry’ by means of his single unifying theory.