Abstract object theory is a branch of metaphysics regarding abstract objects. Originally devised by metaphysicist Edward Zalta in 1999,^[1] the theory was an expansion of mathematical Platonism.

Abstract Objects: An Introduction to Axiomatic Metaphysics (1983) is the title of a publication by Edward Zalta that outlines abstract object theory.^[2]

On Zalta's account, there are two modes of predication: some objects (the ordinary concrete ones around us, like tables and chairs) "exemplify" properties, while others (abstract objects like numbers, and what others would call "non-existent objects", like the round square, and the mountain made entirely of gold) merely "encode" them.^[3] While the objects that exemplify properties are discovered through traditional empirical means, a simple set of axioms allows us to know about objects that encode properties.^[4] For every set of properties, there is exactly one object that encodes exactly that set of properties and no others.^[5] This allows for a formalized ontology.