Metaphysics (Greek words meta=after/beyond and physics=nature) is a branch of philosophy, and related to the natural sciences, like physics, psychology and the biology of the brain; and also to ,mysticism religion, and other spiritual subjects. It is notoriously difficult to define, but for purposes of briefly introducing it, it can be identified as the study of any of the most fundamental concepts and beliefs about the basic nature of reality, on which many other concepts and beliefs rest—concepts such as being, existence, universal, property, relation, causation, space, time, event, and many others.
Part of the trouble with defining metaphysics lies in how much the field has changed since it was first given its name by Aristotle's editors centuries ago (see below). Problems that were not originally considered metaphysical have been added to metaphysics. Other problems that were considered metaphysical problems for centuries are now typically relegated to their own separate subheadings in philosophy, such as philosophy of religion, philosophy of mind, philosophy of perception, philosophy of language, and philosophy of science. It would require quite a long time to state all the problems that have, at one time or another, been considered part of metaphysics.
What might be called the core metaphysical problems would be the ones which have always been considered metaphysical and which have never been considered not metaphysical. What most of such problems have in common is that they are the problems of ontology, "the science of being qua being".
Other philosophical traditions have very different conceptions of the metaphysical problems from those in the Western philosophical tradition; for example, Taoism and indeed, much of Eastern philosophy completely reject many of the most basic tenets of Aristotelian metaphysics, principles which have by now become almost completely internalized and beyond question in Western philosophy, though a number of dissidents from Aristotelian metaphysics have emerged in the west, such as Georg Wilhelm Friedrich Hegel's Science of Logic.
It is sometimes difficult to understand what the issues even are, in metaphysics. It might help to begin with a fairly simple example that will help to introduce the problems of metaphysics.
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Imagine now that we are in a room, and in the middle of the room there is a table, and in the middle of the table there is a big, fresh, juicy, red apple. We can ask many metaphysical questions about this apple. This will, hopefully, help us understand better what metaphysics is.
The apple is an excellent example of a physical object: one can pick it up, throw it around, eat it, and so on. It occupies space and time and has a variety of properties. Suppose we ask: what are physical objects? This might seem like the sort of question to which one cannot give an answer. What could one possibly use to explain what physical objects are? But philosophers actually do try to give some general sorts of accounts of what they are. They ask: Are physical objects just bundles of their properties? Or are they substances which have those properties? That is called the problem of substance or objecthood.
Here is another sort of question. We said that the apple has properties, like being red, being big, being juicy. How are properties different from objects? Notice, we say that things like apples have properties like redness. But apples and redness are different sorts of items, of things, of entities. One can pick up and touch an apple, but cannot pick up and touch redness, except perhaps in the sense that you can pick up and touch red things. So how can we best think about what properties are? This is called the problem of universals.
Here is another question about what physical objects are: when in general can we say that physical objects come into being and when they cease to exist? Surely the apple can change in many ways without ceasing to exist. It could get brown and rotten but it would still be that apple. But if someone ate it, it would not just have changed; it would no longer exist. So there are some metaphysical questions to be answered about the notions of identity, or being the same thing over time, and change.
This apple exists in space (it sits on a table in a room) and in time (it was not on the table a week ago and it will not be on the table a week from now). But what does this talk of space and time mean? Can we say, for example, that space is like an invisible three-dimensional grid in which the apple is located? Suppose the apple, and every other physical object in the universe, were to be entirely removed from existence: then would space, that "invisible grid," still exist? Some people say not—they say that without physical objects, space would not exist, because space is the framework in which we understand how physical objects are related to each other. There are many other metaphysical questions to ask about space and time.
There are some other, very different sorts of problems in metaphysics. The apple is one sort of thing; now if Sally is in the room, and we say Sally has a mind, we are surely going to say that Sally's mind is a different sort of thing from the apple (if it is a sort of thing at all). I might say that my mind is immaterial, but the apple is a material object. Moreover, it sounds a little strange to say that Sally's mind is located in any particular place; maybe we could say it is somewhere in the room; but the apple is obviously located in a particular place, namely on the middle of the table. It seems clear that minds are fundamentally different from physical bodies. But if so, how can something mental, like a decision to eat, cause a physical event to occur, like biting down on the apple? How are the mind and body causally interconnected if they are two totally different sorts of things? This is called the mind-body problem, which is now typically relegated to a philosophical subdiscipline called philosophy of mind. The mind-body problem is sometimes still considered part of metaphysics, however.
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