Existence: I've never really seen this defined, and contemporary analytic metaphysics tends to make no distinction between existing and being. The Meinongians famously distinguished between different kinds of being, where some things with being might not exist.
See Metaphysics: the science of being (according to Aristotle and his tradition) vs the inquiry about the fundamental problems of reality: Space and Time, Causation, Freedom and Determinism, the Mental and Physical (according to modern views). In both case, it is not theology (the "science of God").
Heidegger once delivered a famous lecture called "What Is Metaphysics." After he finished and a baffled silence a student raised his hand. "But Professor Heidegger," he asked, "what is metaphysics?" To which Heidegger responded, "That is a very good question." To this old wives tale I'd add the suggestion that the answer also changes, because "physics" changes.
Metaphysics is a very broad field, and metaphysicians attempt to answer questions about how the world is. Ontology is a related sub-field, partially within metaphysics, that answers questions of what things exist in the world. An ontology posits which entities exist in the world.
Metaphysics is a branch of philosophy concerned with explaining the essence of things, of the fundamental nature of being and the world and the principles that organize the universe.
metaphysics philosophy-of-science philosophy-of-mathematics truth Improve this question edited at 1:07 Julius Hamilton
The difference in the philosophical world is that there is no one, universally acclaimed definition of either Truth or Reality for philosophers. Rather, the nature of these are foundational questions that help distinguish different philosophical positions and traditions. For Plato, for instance, Reality is a deeper level of existence underlying the perceptible world, and Truth is what brings ...