ens realissimum (Latin ‘most real being’), an informal term for God that occurs rarely in Scholastic philosophers. Within Kant’s philosophy, it has a technical sense. It is an extension of Baumgarten’s idea of ens perfectissimum (most perfect being), a being that has the greatest number of possible perfections to the greatest degree. Since ens perfectissimum refers to God as the sum of all possibilities and since actuality is greater than possibility, according to Kant, the idea of God as the sum of all actualities, that is, ens realissimum, is a preferable term for God.
Kant thinks that human knowledge is ‘constrained’ to posit the idea of a necessary being. The necessary being that has the best claim to necessity is one that is completely unconditioned, that is, dependent on nothing; this is ens realissimum. He sometimes explicates it in three ways: as the substratum of all realities, as the ground of all realities, and as the sum of all realities. Ens realissimum is nonetheless empirically invalid, since it cannot be experienced by humans. It is something ideal for reason, not real in experience.
According to Kant, the ontological argument begins with the concept of ens realissimum and concludes that an existing object falls under that concept (Critique of Pure Reason, Book II, chapter 3).
See also BAUMGARTEN, KANT. A.P.M.