Descartes' Meditations puts forth a concept of causality sometimes referred to as his 'heirloom theory of causality.' Normal causality requires that the cause must be similar in nature to the event caused. Descartes admits this and uses it for some of his arguments. For instance, when a pool cue strikes a pool ball the cue can cause the ball to move because they are both material objects. The heirloom theory is meant to solve a problem Descartes encountered because of his dualism. He believed that we have a non-material soul and a that this non-material soul could be the cause of the movements of our material body. But since normal causality does not allow a non-material substance to interact with a material substance, Descartes suggested that the effect inherits something from the cause. Most philosophers find this theory unsatisfying.

In his Rules for the Direction of the Mind, Descartes says, "The purely intellectual objects are those that the understanding knows by means of an innate light, without the help of any corporeal image." He goes on to list some of these purely intellectual objects as "the nature of knowledge, doubt, ignorance, or the action of the will (which we may call volition), or the like ... we need only have attained to a share of reason in order to do so." So what what he seems to be saying is that some things such as reason alone gives us the ability to know some things. Therefore it is what our reason can understand without outside help (what he calls a corporeal image). We cannot know anything about 'corn' without seeing some material thing called corn.