The chapter explores how the Stoics account for the totality of movement in the cosmos – in other words, the agency of the world. The Stoics take the world to be a complex and divine living-being. Their physics combines what today we might call physics, biology and theology. The Stoics put forward two premises that appear to be in tension. First, they claim that the active principle is the sole source of movement and cause of everything. Second, they offer a scala naturae according to which kinds of entities differ by the way in which they move and jointly co-cause all movement in the world. This One-Many Problem is the Stoic version of what is later called the problem of free will and determinism, or so I argue. As the Stoics conceive of the problem, the challenge consists in showing how both premises – One Cause and Many Causes – are true. The Stoic approach strikes me as attractive, both because it looks at humans together with animals and other parts of the world and because of its upshot for human agency. Our reasoning, including our decision-making, constitutes some of the causes that co-cause the world’s overall movements. What remains puzzling, however, is that our reasoning is subject to norms. Ultimately, I argue, the puzzle is how norms for practical reasoning fit into the physical world. This reconstruction does justice to the evidence. It recognizes that the Stoics address human movement in the context of a scala naturae. And it predicts what indeed we find: a wide range of texts about the status of human assent, indicative of the awareness that this is not an easy topic.

If the sceptic holds nothing to be true, his dogmatic opponent argues, he is not able to act. This is the core of the famous Apraxia challenge, arguably the best known anti-sceptical argument in antiquity. Several versions of this argument figure in ancient scepticism. At least for the most part, these arguments rely on core assumptions of Stoic epistemology and theory of action: that we have impressions (phantasiai), some of which are practical, i.e. prescribe an action as to be done, and that we either assent to our impressions or not. In assenting to an impression, we hold something to be true. For fools (i.e. almost everyone), this amounts to having an opinion. For the wise person, every instance of holding something to be true is a piece of knowledge. In the case of practical impressions, assent (sunkatathesis) is further identified with impulse (hormê). If there are no external impediments, impulse sets off action.

The sceptic’s explanation of his actions most importantly consists in his response to the Apraxia challenge, and more particularly, in his critical engagement with the Stoic claim that, in action, we assent and thus hold something to be true. Our interpretation of his response to the Apraxia challenge, however, relates to a broader framework of questions. First, how closely does the sceptic’s life resemble ordinary life? Are the sceptic’s actions, in the end, like everybody’s actions? Secondly, does the sceptic consider his way of life as an attempt at living well? Is suspension of judgement, perhaps, a safer road to happiness than adherence to a (potentially false) philosophical account of what is good and bad?