Galen is well known as a critic of Stoicism, mainly for his massive attack on Stoic (or at least, Chrysippean) psychology in "On the Doctrines of Hippocrates and Plato" (PHP) 2-5. Galen attacks both Chrysippus' location of the ruling part of the psyche in the heart and his unified or monistic picture of human psychology. However, if we consider Galen's thought more broadly, this has a good deal in common with Stoicism, including a (largely) physicalist conception of psychology and a (...) strongly teleological view of natural entities, shared features which are acknowledged in several treatises outside "PHP." Why, then, is Galen such a remorseless and negative critic of Stoicism in "PHP"? Various factors are relevant, including the shaping influence on Galen of the Platonic-Aristotelian (part-based) psychological framework. But, it is suggested here, an important underlying factor is the contrast between two ways of thinking about the part-whole relationship, a 'composition' and a 'structure' approach or an atomistic and holistic approach. This contrast is most evident and explicit in one section of "PHP" 5, where Galen, criticising Chrysippus' holistic psychology, denies that the Stoic thinker is entitled to use the concept of part at all. But the contrast is also seen as pervading Galen's response to Stoic thought more generally, in "PHP" and elsewhere, in ways that inform his explicit disagreements with Stoic theory. Stoicism is presented here as having a consistently 'structure' (or holistic) approach. Galen's approach is seen as more mixed, sometimes sharing, or aspiring towards, a holistic picture, and yet sometimes (especially in "PHP" 5), adopting a strongly 'composition' or atomistic standpoint. This (partial) contrast in conceptual frameworks is presented as offering a new perspective on Galen's critique of Stoic psychology in "PHP" and on his relationship to Stoic thought more generally. (shrink)