Metaphysics is the study of the most general features of reality, including existence, objects and their properties, possibility and necessity, space and time, change, causation, and the relation between matter and mind. It is one of the oldest branches of philosophy.

The word metaphysics has its origin in the ancient Greek words metá (μετά, meaning 'after', 'above', and 'beyond'') and phusiká (φυσικά), as a short form of ta metá ta phusiká, meaning 'what comes after the physics'. 

Metaphysics is the branch of philosophy that examines the basic structure of reality.

It is traditionally seen as the study of mind-independent features of the world, but some theorists view it as an inquiry into the fundamental categories of human understanding. Some philosophers, including Aristotle, designate metaphysics as first philosophy to suggest that it is more fundamental than other forms of philosophical inquiry.

Metaphysics encompasses a wide range of general and abstract topics. It investigates the nature of existence, the features all entities have in common, and their division into categories of being. An influential division is between particulars and universals. Particulars are individual unique entities, like a specific apple. Universals are general features that different particulars have in common, like the color red. Modal metaphysics examines what it means for something to be possible or necessary. Metaphysicians also explore the concepts of space, time, and change, and their connection to causality and the laws of nature. Other topics include how mind and matter are related, whether everything in the world is predetermined, and whether there is free will.

Ontology uses diverse methods of inquiry. They include the analysis of concepts and experience, the use of intuitions and thought experiments, and the integration of findings from natural science. 

Formal ontology is the branch of ontology investigating the most abstract features of objects. Applied ontology employs ontological theories and principles to study entities belonging to a specific area. For example, social ontology examines basic concepts used in the social sciences. Applied ontology is of particular relevance to information and computer science, which develop conceptual frameworks of limited domains. These frameworks are used to store information in a structured way, such as a college database tracking academic activities. Ontology is relevant to the fields of logic, theology, and anthropology.

The origins of ontology lie in the ancient period with speculations about the nature of being and the source of the universe, including ancient Indian, Chinese, and Greek philosophy. In the modern period, philosophers conceived ontology as a distinct academic discipline and coined its name.

Epistemology is the branch of philosophy that examines the nature, origin, and limits of knowledge.

Also called theory of knowledge, it explores different types of knowledge, such as propositional knowledge about facts, practical knowledge in the form of skills, and knowledge by acquaintance as a familiarity through experience. Epistemologists study the concepts of belief, truth, and justification to understand the nature of knowledge. To discover how knowledge arises, they investigate sources of justification, such as perception, introspection, memory, reason, and testimony.