...his writings, Descartes made use of three principal arguments. The first (at least in the order of presentation in the Meditations) is a causal argument. While its fullest statement is in Meditation III, it is also found in the Discourse (Part IV) and in the Principles (Part I §§ 1718). The argument begins by examining the thoughts contained in the mind, distinguishing between the formal reality of an idea and its objective reality. The formal reality of any thing is just its actual existence and the degree of its perfection; the formal reality of an idea is thus its actual existence and degree of perfection as a mode of mind. The objective reality of an idea is the degree of perfection it has, considered now with respect to its content. (This conception extends naturally to the formal and objective reality of a painting, a description or any other representation.) In this connection, Descartes recognized three fundamental degrees of perfection connected with the capacity a thing has for independent existence, a hierarchy implicit in the argument of Meditation III and made explicit in the Third Replies (in response to Hobbes). The highest degree is that of an infinite substance (God), which depends on nothing; the next degree is that of a finite substance (an individual body or mind), which depends on God alone; the lowest is that of a mode (a property of a substance), which depends on the substance for its existence. Descartes claims that it is manifest by the natural light that there must be at least as much reality in the efficient and total cause as in the effect of that cause' . From that he infers that there must be as much formal reality in the cause of an idea as there is objective reality in the idea itself. This is a bridge principle that allows Descartes to infer the existence of causes from the nature of the particular ideas that are in the mind, and...
