In the Third Meditation, Descartes proposes the following as “manifest by the light of nature”: “There must be at least as much in the efficient and total cause as in the effect of that cause” (AT VII 40, CSM II 28). Elsewhere he labels this as the “axiom or common notion” that “whatever is of reality or perfection in some thing is formally or eminently in the first and adequate cause of it” (AT VII 165, CSM II 116).

This “containment axiom,” including its appeal to the “formal or eminent containment” of the reality of an effect in its “efficient and total cause,” is straight from the Scholastic tradition. Descartes himself offers technical definitions of the two different kinds of containment, noting that things “are said to be formally in the objects of ideas, when they are such as we perceive them [talia sunt in ipsis qualia illa percipimus], and eminently, when they are not such [as we perceive], but so great that they can take the place of such things [that are such as we perceive] [quando non quidem talia sunt, sed tanta, ut talium vicem supplere possint]” (AT VII 161, CSM II 114).

In the case of formal containment, the definition has a distinctively mentalistic cast, as reflected in its reference not only to what is in the “objects of ideas” but also to features of objects that are “such as we perceive them.” This connection between our perceptions or ideas and their objects is further mediated by the objective reality of an idea, which Descartes defines – just before defining formal and eminent containment – as “the entity of the thing represented by the idea [entitatem rei repraesentatae per ideam], insofar as it is in the idea…. For whatever we perceive as in the objects of ideas, they are in the ideas themselves objectively” (AT VII 161, CSM II 113–14) (see being, formal versus objective). For Descartes, then, the paradigmatic case of formal containment is one in which the entity as it exists outside of our idea is “such as” the reality insofar as it is represented by our idea.

Descartes’ definition of eminent containment indicates that this sort of containment is supposed to accommodate cases where the cause differs in nature from its effect, and so cannot contain this effect “such as we perceive it.” However, this definition does not fit very well his own example in the Sixth Meditation of this sort of containment.