All substances also have affections, things that hold of the substances. Affections are not substances themselves because they cannot exist independently of the substances that they affect. Affections can be divided between attributes and modes, and attributes can be divided between primary attributes and omni-generic attributes. Primary attributes are those attributes that explain the essence of the substance they belong to. According to Descartes, the primary attribute of body is extension and the primary attribute of mind is thought. Thus, all bodies are necessarily extended in space and all things that are extended in space are necessarily bodies. Similarly with minds and thought: all thinking things are minds, and all minds think. Omni-generic attributes are attributes that can hold of any substance and do not define their essence. Examples are existence, duration, and number. Modes are modifications of primary attributes, ways in which something can have a certain primary attribute. For instance, squareness is a mode of extension since it is a way in which a body might be extended. Similarly with color, size, other shapes, motion, etc., for bodies; and imagining, willing, sensing, feeling, etc., for minds.
Now we move on to the distinction between formal and objective reality. For Descartes and the Scholastics, ideas are the link that connect mind and world because they have both formal and objective reality. To clarify the distinction once more, formal reality is the kind of reality things have in this world and objective reality is the reality of the objects represented by different ideas. Thus, an idea can have formal reality, being a mode of thought itself, and it can also have objective reality, representing something outside of itself.
When Descartes speaks of things as having more or less reality than other things, we can understand him as roughly dividing up reality along a scale where infinite substances (i.e., God) have the most reality, followed by finite substances, followed by modes. As we mentioned earlier, finite substances are bodies and minds, while modes are modifications of body and mind, like color, shape, size, imagination, idea, will, etc. This implies, among other things, that ideas have the formal reality of modes, since they are modifications of mind. So, for instance, the idea of a car would have the formal reality of a mode (since it is an idea) and the objective reality of a finite substance (since the idea is of a car, which is a body). On the other hand, the idea of the fear of cars would have the formal reality of a mode (since it is an idea) and the objective reality of a mode (since the idea is of a fear, and fear is also a mode of thought).
According to Descartes, something with a certain degree of objective reality must ultimately be caused by something with that degree of formal reality. So, for instance, the idea of a car (which has the objective reality of a finite substance) might be caused by the idea of a bicycle, which only has the formal reality of a mode, but that idea of a bicycle might then have been caused by a bicycle itself, which has the formal reality of a finite substance. If we trace the causal chain far enough back, we will find a cause with as much formal reality as the idea has objective reality. If the Meditator can locate an idea that has more objective reality than he has formal reality, he can conclude that there must exist something outside of himself which had to create the idea. Since he has the formal reality of a finite substance, the only thing that has more reality is infinite substance. Thus, he will try to prove that something besides himself exists by contemplating his idea of God.