Deleuze's use of thermodynamics in the fifth chapter of his masterwork Difference and Repetition ushers in perhaps the most crucial notion for understanding this work: intensity. Given that the process of actualisation relies on the intensive necessarily means that any discussion of the relationship between the virtual and the actual must include a thorough explanation of the role of intensity, and where exactly this notion sits within the virtual-actual doublet. As such, we must return to the fifth chapter of Difference and Repetition in order to assess the way in which Deleuze conceives of this complex and indispensable cog in his ontological and metaphysical philosophy. In this paper we turn, then, to Deleuze's engagement with the theories of the intensive as found in classical thermodynamic theory. Quite simply, Deleuze will highlight the way in which the productive force of differences in intensity remains under-appreciated in contrast to the extensive states they create in this classical framework. We can understand this engagement, then, as an example of the way in which the extensive is continually privileged over the intensive. In this paper we will outline the details of the engagement between Deleuze and thermodynamics, as this paves the way for his own philosophy of intensity. Furthermore, we will provide a theory of the relationship between intensity, the virtual and the actual as these notions appear in Difference and Repetition