Agency is the philosophical and psychological idea there are entities that act with intentions and purposes and are causally responsible for future events.

Agent-Causality is the (perhaps metaphysical?) idea that agents can start new causal chains that are not pre-determined by the events of the immediate or distant past and the physical laws of nature.

The first agent-causal libertarian was Aristotle, followed by Epicurus, and then Carneades.

In more recent times, prominent agent-causalists have been Thomas Reid in the 18th century, followed by Roderick Chisholm, Richard Taylor, Keith Lehrer, Timothy O'Connor, and Randolph Clarke in the 20th century.

Aristotle was in many ways the architect of causality as well as agent-causality.

First he described a causal chain back to a prime mover or first cause, and he elaborated the four possible causes (material, efficient, formal, and final). Aristotle's word for these causes was ἀιτία, which translates as causes in the sense of the multiple factors responsible for an event. Aristotle did not subscribe to the simplistic "every event has a (single) cause" idea that was to come later.

One generation after Aristotle, Epicurus argued that as atoms moved through the void, there were occasions when they would "swerve" from their otherwise determined paths, thus initiating new causal chains. Epicurus argued that these swerves would allow us to be more responsible for our actions, something impossible if every action was deterministically caused. For Epicurus, the occasional interventions of arbitrary gods would be preferable to strict determinism.

Epicurus did not say the swerve was directly involved in decisions. His critics, ancient and modern, have claimed mistakenly that Epicurus did assume "one swerve - one decision" and that "free " actions are uncaused.

But following Aristotle, Epicurus thought human agents have the autonomous ability to transcend necessity and chance (both of which destroy responsibility), so that praise and blame are appropriate. This is a form of agent-causality. It answers the flawed standard argument against free will..

Three causes -
Necessity
Chance
Autonomous Agents

...some things happen of necessity, others by chance, others through our own agency. ...necessity destroys responsibility and chance is uncertain; whereas our own actions are autonomous, and it is to them that praise and blame naturally attach.
(Letter to Menoeceus, §133)

The Stoics and later the Christian church censored much of Epicurean thought and destroyed Epicurus' reputation as a "hedonist" and an atheistic believer in "chance." They led to the modern impression that Epicurus thought his chance "swerve" was his basis for free will. Philosophers today are astonishingly ignorant of Epicurus' actual position on agent autonomy. Can we correct this?

Alexander of Aphrodisias (c.150-210), the most famous commentator on Aristotle, writing 500 years after Aristotle's death, defended a view of moral responsibility we would call libertarianism today, with a strong sense of agent-causality.

Alexander believed that Aristotle was not a strict determinist like the Stoics, and Alexander himself argued that some events do not have pre-determined causes. In particular, man is responsible for self-caused decisions, and can choose to do or not to do something.

For Alexander, as for Aristotle, a random event "for no reason" provides a fresh start or new beginning (ἀρχή) of a causal chain (ἄλυσις) that can not be traced back indefinitely. This effectively puts an end to the Stoic ideas of foreknowledge and pre-determination.

In particular, he held that man is responsible for self-caused decisions, and can choose to do or not to do something. This appears to be not very different from the Stoic Chrysippus' idea that one can assent or dissent to an action. Chrysippus said actions are pre-determined (fated) but not necessitated.

Alexander denied three things - necessity (ἀνάγκη), the foreknowledge of fated events that was part of the Stoic identification of God and Nature, and determinism in the sense of a sequence of causes that was laid down beforehand (προκαταβεβλημένος) or predetermined by antecedents (προηγουμένος).

Alexander, following Aristotle and Epicurus, saw three main things causing events. They are necessity, chance, and agent-causality - what is "up to us"