 Zalta (1993) tackles this problem on the basis of his axiomatic theory of abstract objects. According to him, abstract objects like numbers exemplify but also encode properties. For example, 1 encodes the properties that are essential to its identity; to its being the object it is. In contrast, 1 exemplifies, but does not encode, the property of being the number of inhabited planets of the solar system. In this system, situations are those abstract objects that encode state of affairs properties such as being such that John is to the left of Harry. If a situation is the case, the state of affairs properties it encodes obtain; the situation makes the encoded state of affairs property factual. A situation S is a possible world if, and only if, it is possible that S makes all and only the obtaining states of affairs properties factual. On the basis of the axioms of his system Zalta can show that there is a unique actual world.