I’d like to examine a potential tension between Descartes’ claim in the Principles that (i) the term ‘substance’ “applies univocally to mind and body”  and his claim in the Passions that (ii) “an action and passion must always be a single thing” . In regards to the first claim we are told that mind and body share the designation ‘substance’ as they are the only two things who depend on nothing other than God’s concurrence for their existence. This means that as long as God concurs with its existence, mind can exist apart from body and body apart from mind. Of course, it follows that if one of these two things – either mind or body – were found to depend on the other, then the term ‘substance’ would not apply univocally to both. One would depend only on God, while the other would depend on both God and the prior substance. Descartes calls this third kind of entity a “quality” or “attribute” . The concern is that by identifying actions with passions Descartes renders the body an attribute of the mind.
First, consider the three types of distinction Descartes identifies in the Principles. There are real, modal and conceptual distinctions. A real distinction is drawn between two or more types of substance – e.g. mind and body – if one substance can be “clearly and distinctly understood apart from the other” . A modal distinction can be drawn in two ways. The first way distinguishes a substance from its attribute or mode insofar as the former can be understood without a clear perception of the latter while the latter cannot be understood apart from the former (e.g. body without motion but never motion without body) . The second kind of modal distinction is drawn between two attributes inhering independently in one substance, both relying on the substance but neither relying on each other for their comprehensibility (e.g. weight and motion) . Lastly, a conceptual distinction holds between a substance and some essential attribute of that substance “without which the substance is unintelligible” (e.g. mind and thinking) or between two or more attributes of a single substance each of which cannot be understood apart from the other (e.g. God’s essence and his existence) .
Each of these distinctions seems to be at work in Passions of the Soul where Descartes sets out to, inter alia, establish the real distinction between mind and body by examining the ways in which their functions are related (or not related). If we attend to claim (ii) above — “an action and passion must always be a single thing” — then it is clear that actions and passions, unlike minds and bodies, could not exemplify a real distinction as they are not substances. This leaves modal and conceptual distinction. Given (ii) it seems prima facie evident that the distinction between actions and passions is conceptual. After all, insofar as they represent a “single thing” we cannot, presumably, understand them independently. However, if two substances are really distinct, as mind and body are said to be, then it would seem to follow that their respective attributes must be modally distinct and not merely conceptually distinct, lest we are driven to think one substance whenever we try to comprehend the attributes of the other.
Now, if actions were modally distinct from passions, it would have to be possible to clearly and distinctly perceive one without the other. And although it may appear obvious, on first glance, that we can comprehend actions apart from passions (for example, when we imagine two bodies acting upon each other where action in one gives rise to an action in the other and passion is nowhere involved), this becomes less obvious in light of Descartes’ general postulate that “whatever takes place or occurs is…a ‘passion’ with regard to the subject [qua patient] to which it happens and an ‘action’ with regard to that [subject qua agent] which makes it happen.”  Thus, if a movement of an active subject is to qualify as an action it would appear that that same movement must simultaneously qualify as a passion in the passive subject. And, assuming claim (i) is true — that mind and body rely only on God’s concurrence and neither on each other — this rule should apply equally to body–>mind interactions, mind–>body interactions, mind–>mind interactions and body–>body interactions. Now, we see Descartes applying the rule when the body acts upon the mind (§21), when the mind acts upon the body (§41), and when the mind acts upon the mind (§17-20). But we do not see him applying it to cases of body–>body interaction. Wherever action is purely corporeal, passion is absent. Instead, whenever passion is mentioned it is always in relation to the mind. Passion seems to rely on mind in a way that action does not. In this case, we may want to conclude that actions and passions are indeed modally distinct, but it would then follow that a passion could be understood without an understanding of action, which seems implausible given the passivity inherent in the very concept of the passions.
We have run into a dilemma: either affirm the modal distinction between actions and passions and deny claim (ii) or affirm the conceptual distinction between action and passion and deny claim (i). For the sake of argument let us take the second horn. If actions and passions are merely conceptually distinct then there is no univocal way to apply the term ‘substance’ to both mind and body. This is because, while the actions of the body cannot be clearly understood without thinking of the passions, and the passions only inhere in the mind, the passions of the soul can be understood sufficiently without any understanding of the body. The body is not independently clear and distinct in the way that the mind is. Consider that the mind can act on itself in such a way that its volitional action and the passion through which it is perceived are “one and the same thing” . This is sufficient to show that mind is a substance but that body, insofar as it is capable of action, is an attribute of mind, since bodily action is unthinkable apart from an understanding of the passions of the soul. Body without mind is inert.
Texts: Descartes, Principles of Philosophy,
________, Passions of the Soul,
Rene Descartes, “Descartes: Selected Philosophical Writings”, translated by John Cottingham, Robert Stoothoff, and Dugald Murdoch, Cambridge University Press, 2009, (CSM)
 Paginations appearing in the body of the text follow marginal numeration from CSM.