Metaphysics is a major branch of philosophy. It concerns existence and the nature of things that exist. Altogether it is a theory of reality.

Ontology is the part of metaphysics which discusses what exists: the categories of being. Apart from ontology, metaphysics concerns the nature of, and relations among, the things that exist.

Topics that are discussed in metaphysics include existence, objects and their traits, space and time, cause and effect, and what is possible.

The study of ontology is broad, while other branches of metaphysics are more specific. One main question in ontology are what the most basic categories of being are. For example, imagine a tree. A tree is part of a larger category, like a plant. Plants are part of larger category, too: living things. Eventually, we arrive at a very large category: substance. Philosophers who study ontology want to discover and understand basic categories like substance. Aristotle, for example, tried to understand reality through many other categories that he discovered, such as Quantity, Quality, Relation, Place, and Time among others.

In the 15th century René Descartes thought there might be two different substances, mind and matter. This is a view called dualism. Other thinkers, like Immanuel Kant, thought that we cannot say anything about substance, as the only way we can talk about substance is through connections. To prove this, Kant used the sentence "This is a house". Kant thought that the meaning of the word house depended on how other people used houses, or how other houses looked like. This meant that meaning was all about connections, and that there is no meaning of "house" on its own. So Kant replaced the category of substance with relation in his metaphysical beliefs.

In the 20th century, some Western philosophers thought these questions were really just questions about the definition of words. Ludwig Wittgenstein thought that words do not have clear definitions. Instead, definitions are blurry. For some philosophers, this meant that metaphysics had no use. For other philosophers, this inspired them to think about metaphysics in new ways.^[1][2][3]

Identity is an important metaphysical topic. All the objects in the world seem to always be changing. Heraclitus, an ancient Greek philosopher, famously said: "You cannot step in the same river twice."^[4] The river is always flowing, so talking about the river as if it has remained the same river is impossible. Heraclitus concluded that identity was not real, only change was. Parmenides, another philosopher, thought the opposite: change is not real, and is an illusion. To him, everything has always been the same object. This is a view called monism.^[5]

In the 17th century, Leibniz discovered the Law of Identity of Indiscernibles. The law says that if some object X is identical to some object Y, then object X and object Y must have all the same traits. Philosophers thought that there may be two kinds of traits: intrinsic and extrinsic. Intrinsic traits are the traits that make the object what it really is. Extrinsic traits are traits that are about its relationship with other things. So to solve the problem of change, some philosophers have argued that what is changing are the extrinsic traits, but the intrinsic traits remain the same. So for example, while your body has changed over the years, there may be intrinsic traits to you that have not changed and make you who you really are. This could be a soul, psychological consistency, or something else.^[6]

Objects appear in space and time, and so it is of much interest in metaphysics. Some metaphysical questions about space and time are:

• Does space and time exist independently of the mind?
• Do space and time exist independently of one another?
• Do times other than the present exist?

In the 17th century, Leibniz and Newton had a famous debate about whether space and time were real objects, or if they were just a method of ordering objects. Newton thought space and time were real and absolute. Leibniz thought it must be relative.^[7] This was unsettled for centuries until Albert Einstein proposed that the laws of physics should be based on the principle of relativity, and that space and time could not be absolute.

In the 18th century, Kant argued that space and time are not substances or something we learn from our conscious experience. Instead, Kant thought that space and time were part of our mind's system and allows us to organize our experience of the world. While this meant that space and time were mind dependent, Kant still believed that they were empirically real.^[8]

What do we mean when we say something was caused by something else? This is an important question for scientists as well as philosophers. Causation is a kind of influence where one event helps creates another event. Aristotle believed that causality meant "explanation", and separated causality into four causes: material, formal, efficient, and final. Material cause is what an object is made of. Formal cause is the pattern or shape that organizes the matter in the object. Efficient cause is who or what changes and creates the object. The final cause is the reason why that object was created.^[9] Most philosophers today are interested in talking about efficient cause.

There are many theories on causation. In 1973, David Lewis thought that causality was like a chain of things depending on one another. So, for some event C to cause some event E, that means there is a chain of other events that connect them together.^[10] Other theories use probability. For example, smoking does not always cause cancer, but it increases the chances of it happening. As a result, it is more useful to turn to a definition of causality that uses probability.

Metaphysical cosmology and cosmogony try to address questions such as:

• What is the origin of the Universe? What is its first cause?
• What are the ultimate material components of the Universe?
• What is the ultimate reason for the existence of the Universe? Does the universe have a purpose?

Explaining our minds in a world made of matter is a special metaphysical problem. In the modern, philosophers and scientists began to reject non-physical ideas about reality. This created a view called materialism, which argues that matter is more basic than mind. But it seems our thoughts and perceptions are not material. This makes consciousness hard to explain under the materialist view.

This motivates some philosophers to explore other ideas, such as dualism, idealism, or panpsychism. Dualism argues that mind and matter and two separate things. According to dualism, mind might be like the soul. However, dualists must explain how the soul interacts with the body. Idealism argues that mind is more basic than matter, and that objects do not exist until we sense them. Panpsychism argues that everything has a mental aspect, so using a theory called neutral monism, one can argue that mind and matter are actually the same thing.