 Descartes argues, in general: if a particular quality or affection exists, it must subsist or reside in a substance that makes it exist (CSM 2.114; AT 7.61; CSM 1.210; AT 8.25). The substance must have the capacity to enable this particular quality to exist and any other quality in a range in which this one is included (CSM 2.124; AT 7.76) The range is determined by its principal attribute, or essence: all qualities (or modes) of a substance are referred to its essence and unintelligible without it (CSM 1.210; AT 7.25; CSM 2.54; AT 7.78). So the essence of a substance and all the qualities that do or can exist in it have reciprocal relations of conceptual dependence (Bolton 2013: 65–6). Scholars characterize the relation in several ways—as that between an occult quality and its known effects (Malebranche 1674–75 [1980: 622]), the atomic structure of gold and its sensible and laboratory properties (Rozemond 1998: 92; Beck 1965: 118–22), an absolute and its limits (Downing 2011: 122–25). But Secada (2000: 190–94) cites a letter in which Descartes speaks of the relation as an instance of the more general relation between two concrete things one of which is determined by the other (To Mersenne for Hobbes, 21 April 1641, CSM 3.178–9; AT 3.355) Secada (also Bolton 2013) concludes the relation should be understood as that between a highest order determinable and its determinates.