This paper attempts to reconstruct the views of the Stoic Posidonius on the emotions, especially as presented by Galen’s On the Doctrines of Hippocrates and Plato. This is a well-studied area, and many views have been developed over the last few decades. It is also significant that the reliability of Galen’s account is openly at issue. Yet it is not clear that the interpretative possibilities have been fully demarcated. Here I develop Galen’s claim that Posidonius accepted a persistent, non-rational aspect of the soul that he connects with the merely animal part of humans. The aim is to begin from this testimony in answering two questions: (1) How might the possession of a non-rational element of the soul operate alongside the hêgemonikon (leading-part of the soul) as a source of impulse for Posidonius. (2) How does this persistent animal aspect conform to the Stoic ontological classification found in their scala naturae? I shall argue in response to these that (a) Posidonius distinguished the merely cognitive aspects of the soul from those that are rational, and (b) that the hêgemonikon itself is not to be identified with what is rational. Accepting a persistent non-rational source of emotional impulses allows Posidonius a richer framework for explaining human affective responses and behaviours. I also briefly address Galen’s motivation for the account he offers. It is in view of Posidonius’ approach to Plato’s Timaeus that Galen’s discussion finds its most plausible interpretation.