If substance is the highest category and there is no substance, being, then the unity perceived in all beings by virtue of their existing must be viewed in another way. St. Thomas chose the analogy: all beings are like, or analogous to, each other in existing. This comparison is the basis of his Analogy of Being. The analogy is said of being in many different ways, but the key to it is the real distinction between existence and essence. Existence is the principle that gives reality to an essence not the same in any way as the existence: "If things having essences are real, and it is not of their essence to be, then the reality of these things must be found in some principle other than (really distinct from) their essence." Substance can be real or not. What makes an individual substance – a man, a tree, a planet – real is a distinct act, a "to be", which actuates its unity. An analogy of proportion is therefore possible: "essence is related to existence as potency is related to act."