Introduction to Philosophy/What is Metaphysics
Metaphysics is hard to define. The term itself can mean "beyond physics" but it's tricky to get more precise without inviting controversy. Early use of the term simply referred to the topics covered by the work placed after (hence meta) the Physics in the traditional editing and not commentary of Aristotle's works made by the Greek Peripatetic philosopher Andronicus Of Rhodes. Let's look at what metaphysics is not. Newton's theories about gravity, Einstein's theories of relativity, and most of the work on quantum mechanics are clearly not metaphysics. These theories make specific predictions about observable phenomena. The theories are subject to being disproven because we can compare their predictions to actual results that we can get in the real world. For example, if we were to state that "for every atom in this universe, there is a corresponding atom in a parallel universe" then this is not physics because it cannot be disproven: we cannot presently detect the presence of atoms in parallel universes.
This brings up an interesting problem for metaphysics. The theory that all matter is composed of tiny particles called atoms was considered metaphysics at one time. When people began to invent instruments capable of detecting such tiny things, atomic theory moved out of metaphysics into physics. So metaphysics is always "at the frontier" and some of it may become physics in the future.
Religion is also not metaphysics. Religion is based on a set of convictions that may well make predictions about the world or explain why certain phenomena occur, but certain kinds of change are not allowed without disturbing the integrity of the religion. This is not to say that religions can't change, but their overall integrity has to be preserved to some extent. For example, some religions (such as Greek Mythology) have explained that mysterious phenomena are caused by many different gods. Within the context of such a religion, it would be impossible to change this belief to say that all of the phenomena are caused by a single god. To do this would be essentially to invent a new and contradictory religion.
This raises another interesting problem for metaphysics. It may often tread on sacred ground and come into conflict with religion. Socrates was one casualty of this tendency.
These considerations give us some boundaries, and we can define metaphysics more readily inside those boundaries.
Metaphysics is concerned with explaining the way things "are" in the physical world. It is concerned primarily with 'being as being' i.e. with anything in so far as it has act of existence. However, metaphysics is not concerned with examining the physical properties of things that exist, but is, instead, the study of the underlying principles that give rise to the unified natural world. As such, the statement that "Evil does not exist" is metaphysical because it is a statement that deals with the object 'evil' as opposed to 'good' which is a metaphysical subject, whereas the statement that "all things are composed of atoms, which are in turn composed of electrons, protons, and neutrons" is definitely not metaphysics, utmost a concern of physical sciences.
Metaphysics is a branch of philosophy, and part of the answer to the question "What is Metaphysics" requires us to define the difference between science and philosophy. 'Science' is taken here as empirical sciences or non-empirical sciences. In physical science, it's important for new explanations to make predictions that can be tested by experiments. But this is not a requirement of philosophy, specifically that of metaphysics. Instead, we reduce any philosophical statement to its ultimate concept or propositions. The ultimate concept of metaphysics is being while that of propositions is the principle of contradiction. In the case of the above statement that all things are made up of smaller things and so on to infinity, this explanation is clearly untestable because we'll never have an instrument capable of detecting anything that is infinitely small. But then, metaphysically, one can be sure that it is impossible to have an infinite regression of materiality but must arrive at the ultimate or smallest particle of matter. But then, the philosopher can further ask : What is the ultimate composition of matter?" this time the answer cannot be anymore 'the smallest particle of matter' since the smallest particle of matter is matter also. Thus, Thales' question: 'What is the ultimate stuff?' is a metaphysical question not an empirical one. His question separated physical science from philosophy. Aristotle offered an answer: his hylomorphic doctrine which states that any material reality is ultimately composed of prime matter (not the matter as we know it) and substantial form.
Metaphysics is not religion because religion involves act of faith, faith guiding reason. In case of metaphysics, it limits its certitude on reason alone. So in summary, here is one answer to the question "What is Metaphysics?":
Metaphysics is not the branch of philosophy that explains physical phenomena using reason and logic in a way that falls outside the bounds of either religion or science' rather it is a philosophical science which deals with transcendental concepts such as being, one, true and good which in its simplest form is 'being as such'. What really makes Metaphysics hard to define is its object: being. Being cannot be defined properly but only descriptively.