Action theory is an area in philosophy concerned with theories about the processes causing willful human bodily movements of more or less complex kind. This area of thought has attracted the strong interest of philosophers ever since Aristotle's Nicomachean Ethics (Third Book). With the advent of psychology and later neuroscience, many theories of action are now subject to empirical testing.

Philosophical action theory, or the philosophy of action, should not be confused with sociological theories of social action, such as the action theory established by Talcott Parsons.

What is left over if I subtract the fact that my arm goes up from the fact that I raise my arm?

Basic action theory typically describes action as behavior caused by an agent in a particular situation. The agent's desires and beliefs (e.g. my wanting a glass of water and believing the clear liquid in the cup in front of me is water) lead to bodily behavior (e.g. reaching over for the glass). In the simple theory (see Donald Davidson), the desire and belief jointly cause the action. Michael Bratman has raised problems for such a view and argued that we should take the concept of intention as basic and not analyzable into beliefs and desires.

Action may refer to:

Philosophy is the study of general and fundamental problems, such as those connected with reality, existence, knowledge, values, reason, mind, and language. Philosophy is distinguished from other ways of addressing such problems by its critical, generally systematic approach and its reliance on rational argument. The word "philosophy" comes from the Greek φιλοσοφία (philosophia), which literally means "love of wisdom".

The introduction of the terms "philosopher" and "philosophy" has been ascribed to the Greek thinker Pythagoras. The ascription is said to be based on a passage in a lost work of Herakleides Pontikos, a disciple of Aristotle. It is considered to be part of the widespread body of legends of Pythagoras of this time. "Philosopher" was understood as a word which contrasted with "sophist" (from sophoi). Traveling sophists or "wise men" were important in Classical Greece, often earning money as teachers, whereas philosophers are "lovers of wisdom" and not professionals.

The main areas of study in philosophy today include epistemology, logic, metaphysics, ethics, and aesthetics.