This paper aims to assess the epistemological value of the notion of sympathy in Galen’s thought. It first shows that Galen makes use of two different concepts of sympathy: the ambient concept, according to which the human body manifests a harmonious part-whole relationship, and the technical concept, with which one connects two pathologies on the basis of the fact that the cause of the ailment is elsewhere than where it surfaces. Neither of these sympathies constitutes in itself a causal explanation (...) of reality. The first merely acknowledges an obvious interrelationship, the second acts as an epistemic tool to shift the focus of diagnosis away from the place where the condition manifests itself. However, the two concepts are not absolutely unrelated; and they come together when the sympathetic paradigm is used to understand the unifying action of the psychic and natural faculties. Very significantly, Galen then provisionally defines the cause as a power or faculty (δύναμις) not out of theoretical necessity but for practical reasons. (shrink)