  Holbach’s choice of example reflects a likely familiarity with criticisms of the Lockean basis for the distinction between primary and secondary qualities found in the writings of Berkeley and Holbach’s friend and correspondent, Hume. Both of these authors use the example of the feeling of pain in heat (Berkeley, Three Dialogues I; Hume Treatise 1.4.4, 3) as a first step in demonstrating the mind-dependence of all properties of body alike and so of casting into doubt any supposed difference in kind between primary and secondary qualities.