 Compatibilists maintain that free will (and/or moral responsibility) is possible even in a deterministic universe. Versions of compatibilism have been defended since ancient times. For example, the Stoics—Chryssipus, in particular—argued that the truth of determinism does not entail that human actions are entirely explained by factors external to agents; thus, human actions are not necessarily explained in a way that is incompatible with praise and blame (see Bobzien 1998 and Salles 2005 for Stoic views on freedom and determinism). Similarly, philosophers in the Modern period (such as Hobbes and Hume) distinguished the general way in which our actions are necessitated if determinism is true from the specific instances of necessity sometimes imposed on us by everyday constraints on our behavior (e.g., physical impediments that make it impossible to act as we choose). The difference is that the necessity involved in determinism is compatible with agents acting as they choose to act: even if S’s behavior is causally determined, it may be behavior that she chooses to perform. And perhaps the ability that matters for free will (and responsibility) is just the ability to act as one chooses, which seems to require only the absence of external constraints (and not the absence of determinism).