Reductionism is a concept in philosophy that claims a description of properties in a complex system can be "reduced" to the lower-level properties of the system's components. For example, the laws and properties of chemistry can be reduced to the laws of physics.

More specifically, the properties of molecules can be reduced to those of atoms, the properties of biological cells can be reduced to those of molecules, plants and animals can be reduced to those of cells, and mind can be reduced to neurons in the brain.

Beyond the properties, reductionists claim that causal laws of nature in the base level must causally determine the laws of a higher level. These thinkers usually have a highly simplistic, materialistic, and deterministic view of the most fundamental laws of nature, namely the laws of classical physics, or the interpretations of quantum physics that deny indeterminism.

Anti-reductionists deny claims that deterministic causal laws can in principle reduce everything, including life and mind, to the fundamental particles of physics. They include emergentists, who think at least some higher level properties and laws cannot be reduced, but must emerge as sui generis entities that need new explanations. They also include vitalists, who believe that a dualistic, non-physical, immaterial substance is needed to explain life, mind, and consciousness.

For example, Jaegwon Kim, the leading analyst of supervenience (as a non-reductive physicalism that might explain mental causation) puts it this way:

The most fundamental tenet of physicalism concerns the ontology of the world. It claims that the content of the world is wholly exhausted by matter. Material things are all the things that there are; there is nothing inside the spacetime world that isn't material, and of course there is nothing outside it either. The spacetime world is the whole world, and material things, bits of matter and complex structures made up of bits of matter, are its only inhabitants.

(Physicalism, Or Something Near Enough. Princeton University Press (2005), p.150)

The original reductionists were the ancient Greek materialists, Democritus and Leucippus, who argued that the world consists only of "atoms and the void." But their fellow materialist, Epicurus, had added an element of chance to provide still more control and moral responsibility, and indeed agency, than physical determinism alone can provide. He said, in his Letter to Menoeceus, §134,

It is better to follow the myth about the gods than to be a slave of the "fate" of the physicists: for the former suggests a hope of forgiveness, in return for honor, but the latter has an ineluctable necessity.

ἐπεὶ κρεῖττον ἦν τῷ περὶ θεῶν μύθῳ κατακολουθεῖν ἢ τῇ τῶν φυσικῶν εἱμαρμένῃ δουλεύειν· ὁ μὲν γὰρ ἐλπίδα παραιτή- σεως ὑπογράφει θεῶν διὰ τιμῆς, ἣ δὲ ἀπαραίτητον ἔχει τὴν ἀνάγκην

It is critically important to note that there is very little new of importance in Epicurus - except the specific mechanism of the atomic swerve - that was not already there in Aristotle's treatment of moral responsibility in the Nichomachean Ethics and the Metaphysics.

Aristotle had already argued against the atomists' necessity (Leucippus) and causal determinism (Democritus). He did not care for the atomists' atheistic dismissal of the gods, but he unequivocally endorsed chance, itself an atheistic idea flying in the face of the gods' foreknowledge, as the specific means of breaking the causal chain of determinism and necessity.