For the use of the term libertarianism in politics, see libertarianism (politics).

In philosophical debates about free will and determinism, libertarianism is generally held to be the combination of the following beliefs:

1. that free will is incompatible with determinism
2. that determinism is false, and
3. that human beings do possess free will.

All libertarians subscribe to the philosophy of incompatibilism which states that an action cannot be both free and physically predetermined. Free actions are ones which could have been different. Traditionally, this has meant that there is no causal chain that necessitated the action prior to the agent freely choosing it; the agent herself is an originator of causal chains.

However, because of metaphysical worries about this "godlike power" [1], there has arisen the alternative idea that, although the universe is indeterministic, an agent's choice is nevertheless caused by previous events, but those events only assign certain probabilities to her choice (e.g. a 30% chance she will do act A, a 70% chance she will not).

The major objection to libertarianism is that it remains a mystery why an agent makes the choice she does - any explanation of the choice (beyond a probabilistic one) would seem to make it determined. However, according to David Hume, if a choice is not determined then it is simply a random event, which is problematic since such a choice would lack purpose.

Although quantum mechanics provides some reason for thinking that determinism may indeed be false, Roy C. Weatherford (in the Oxford Companion to Philosophy) echoes Hume on randomness:

The random behaviour of atoms certainly does not by itself make for the freedom and moral responsibility asserted by libertarians.