The three main branches of philosophy Metaphysics, Ontology and Epistemology are concerned with the question of how science is to be understood philosophically and how we can know anything. While the term of metaphysics was coined in antiquity, the term ontology first appeared in the 17th century with Rudolf Goclenius who separated it from metaphysics. Epistemology has a long history beginning with the ancient Greek philosophers of Socrates and Plato, and continuing with Kant’s „transcendental idealism” to modern times with Peirce and Husserl.
„He [man] is an animal metaphysicum.” Arthur Schopenhauer: The World as Will and Representation. II, 17.
„[...] the question which, both now and of old, has always been raised, and always been the subject of doubt, viz. what being is, is just the question, what is substance?” Aristotle: Met. VII 1, 1028b2-4
The term metaphysics derives from the Greek expression τὰ μετὰ τὰ φυσικά which means „that which is after or beyond physics”. It has originally been Andronicus of Rhodes who catalogued Aristotelian writings in the 1. century B.C and who listed a collection of treatises about being, substance and causality after the Physics: hence μετά φυσικός. It is both a bibliographical designation and a philosophical one, since metaphysics, due to its content, is to be located beyond the physical as the physical world of experience: μετά here means “transcendent”. According to Plato, this is how forms are to be understood as being unchanging, immortal and transpiring the physical world.
For Aristotle, metaphysics is the study of being qua being (ὄντως ὄν). It is about what actually exists, about its attributes insofar as they exist. This does not refer to individual spheres of being (individual sciences), but to being as such: Metaphysics understood as a universal science, as the first philosophy since it is the fundamental exploration of first principles and causes. As such it explores reality.
Ontology is the sub-discipline of metaphysics as its prominent part because it is the science of what there is (ὄντος λόγος). It deals with the categorical1 structure of being such as the general features and relations of things. Alexander Gottlieb Baumgarten denotes ontology as metaphysica generalis in the 18th century, whereas rational theology, psychology and cosmology are aligned with metaphysica specialis.
Every one of us has his own subjective, ontological list2 of things that exist. One may believe in aliens, the other in God. Thus, aliens are on the ontological list of person A, whereas God is on the ontological list of person B. Both aliens and God are not physical but theoretical beings.
This is where Kant’s Copernican revolution comes in insofar as he determines the subject as central to knowledge. For Kant, it is the human mind that creates objects of cognition. The prerequisites for knowledge lie within the human mind and thus in the subject himself. The subject consolidates representation to knowledge via categories of understanding such as time and space. In Kant’s way of thinking, it is the agent that is the starting point of ontology. The innate categories of the human mind construct the experience and thus the scope of knowledge.
In Being and Time (§4, p.1), Heidegger makes the distinction between „ontical” and „ontological” by defining „ontical” as concerned with the concrete entities or what actually is in time and space (Being) and by defining „ontological” as the metaphysical meaning of being (Being as such). From this, it follows that the ontological Being underlies the ontical Being. This is the so-called „ontological difference”3 according to Heidegger.
Epistemology (from Greek ἐπιστήμη meaning knowledge, understanding, science) is concerned with questions about our knowledge of reality and about the structure of knowledge such as its conditions, possibilities and limits. How do we know anything? And is it justified to equal knowledge with true belief? Or as Kant put it in Critique of Pure Reason, B 833: What can I know? What are the sources of knowledge? The mind, reasoning or experience? Knowledge is innate („Forms”) according to the rationalists (Plato and Descartes), whereas the empiricist such as Locke and Hume purport that all knowledge stems from experience (concept of tabula rasa). Aristotle disagreed with Plato’s radical rationalism and defined man as a sensual being that collects impressions (sense perception) and thereby disclosing reality with the help of reason. Aristotle criticised Plato’s Theory of Forms as „empty words” and „poetical metaphors”.4