The aim of this paper is to examine the considerations on stochastic arts in Antiquity and to show how Galen’s analysis concerning the “art of conjecturing” constitutes a preferable alternative to the traditional ways used by philosophers to explain the inherent fallibility in the medical art. By distinguishing the scientific diagnosis from the conjectural one, Galen encompasses all cases relevant to the medical art. The former, because of its general nature, can be theorized. As for the latter, it concerns only (...) the individual and is therefore not likely to enter the theory, except by the method that makes it possible. This paper attempts to outline the modus operandi underlying the technical conjecture in order to illustrate Galen’s deftly developed position on a subject almost exclusively investigated by philosophers until then. (shrink)