Existence is a foundational concept in metaphysics and ontology concerned with what it means for something to be real or to have being. Philosophical questions about existence examine whether all things exist in the same way, how physical objects differ from abstract entities or mental states, and whether concepts such as possibility, necessity, identity, and reality describe different modes or conditions of existence. Discussions of existence often overlap with questions about being, essence, substance, and the categories of things that may be said to exist.