Rather than simply ask "Do abstract entities like numbers and properties exist," a metaphysicist prefers to ask in what way they might exist that is different from the way in which "concrete" objects exist.

Concrete objects can be seen and touched by our senses. They are material, with causal relations that obey the physical laws of nature.

Abstract entities are immaterial, but some of them can still play a causal role, for example when agents use them to decide on their actions, or when chance events (particularly at the quantum level) go this way instead of that.

Just as the mind is like software in the brain hardware, the abstract information in a material object is the same kind of immaterial stuff as the information in an abstract entity, a concept or a "non-existent object." Some philosophers say that such immaterial things "subsist," rather than exist.

Broadly speaking, the distinction between concrete and abstract objects corresponds to the distinction between the material and the ideal. Ideas in minds are immaterial. They need the matter of the brain to be embodied and some kind of energy to be communicated to other minds. But they are not themselves matter or energy. Those "eliminativists" who believe the natural world contains only material things deny the existence of ideas and immaterial information.

Some ideas may be wholly fictitious and nonsensical, whether mere possibles or even impossibles, but most ideas correspond to actual objects or processes going on in the world. In either case, we can usually specify the informational content of the idea.

Metaphysicists identify abstract entities with the information contained in them. They may be concepts that did not exist in the world until they were invented. Or the information may have existed in material structures and so we say they were discovered. For example, the idea of the moon includes the concepts of a distinct shape, color, and even the appearance of a face.

Many such ideas are mind-independent. Consider properties of the moon. Most observers agree the shape is round and the color is white. (Actually, the moon is blacker than most any terrestrial black object. It only appears white compared to the blackness of space.) Some metaphysicians deny the existence of a universal property such as roundness or whiteness. But metaphysicists see the information needed to specify circularity and the wavelengths of radiation that correspond to whiteness. And that information is embodied in the moon, just as a software program is embodied in computer hardware, and a mental idea is embodied in a brain.

Many ideas or concepts are created by human minds by "picking out" some of the information in physical objects. Whether such concepts "carve nature at the joints" (Plato, Phaedrus, 265e) depends on their usefulness in understanding the world.

Plato's Theory of the Forms held that Ideas like the circle pre-exist material beings, where Aristotle argued that the Ideas are abstractions from the most general properties in all the actual circles.

Information philosophy restores so-called "non-existent objects" to our ontology. They consist of the same kind of information that provides the structure and process information of a concrete object. What we call a "concept" about an object is some subset of the information in the object, accurate to the extent that the concept is isomorphic to that subset. By "picking out" different subsets, we can sort objects.

Information philosophy settles deep philosophical issues about absolute and relative identity. All material objects are self-identical, despite concerns about vague boundaries. All objects have relations with other objects that can be interpreted as relative identities. All objects are identical to other objects in some respects and different qua other respects.

In modern times, many philosophers distinguish a third realm beyond the ancient idealism/material dualism. Beginning with early analytic language philosophy, the apparently mind-independent ideas were described as "objective" or "intersubjective" by contrast with the purely "subjective." See the "triads" of Gottlob Frege, Charles Sanders Peirce, Karl Popper, and others.

For Popper, this third realm includes all human knowledge and culture, including human artifacts. We call this the sum of human knowledge.The ideas in our books are not the ink and paper they are printed on.

We could also widen the definition to include the biological realm. It would include the genetic content of all living things, the product of four billon years of evolution. The genetic information is not the nucleotides of DNA that carry it. Both kinds of knowledge, human and biological, are abstract entities.

Human knowledge (information) and biological knowledge are created, stored, and communicated by similar means. New information requires chance events. Storage requires embodiment of abstract symbols or patterns in material information structures.

Communication of those symbols requires transmission through a medium, via sound and sight at a distance, or touch, smell, and taste by contact. These all are evolutionary refinements of the chemical interactions inside living things. Assembled from arbitrary symbols, the syntax and semantics of messages from a cell nucleus to the ribosomes, or messages between cells, even hormonal signaling from the amygdala to the prefrontal cortex, are the progenitors of human prose and poetry.

Many centuries ago, the neoplatonist philosopher Porphyry asked what some called his "fateful question, "what is the existential status of the Platonic ideas?" Metaphysicists see ideas as the information they contain. They have no existence as material, although they might be embodied in material, as its organization. The information can be communicated in the form of energy to other material things.

Many philosophers of mind are "physicalists" or "eliminative materialists." The mind and mental events are described as redundant causes that can be excluded, since them material brain already provides physical events as the cause.