The chapter deals with Galen’s attack on the pulse-classification of the first-century CE Pneumatist doctor Archigenes, and examines Galen’s reasons for replacing Archigenes’ theory with his own. Galen claims that Archigenes has no idea of the proper method of determining the real species of pulse, and he castigates him for terminological failings as well. But it turns out on close analysis that Archigenes’ actual classification is very close to Galen’s own; and the terminological cavils seem fairly trivial and pedantic. So what is the real substance of Galen’s attack? The chapter suggests that the point at issue is partly simply a matter of professional rivalry, but partly also a consequence of Galen’s insistence on adhering to the properly philosophical method of conducting divisions.

This chapter discusses a number of passages from Galen’s work that illustrate his idea of dialectic as a tool for scientific reasoning and discovery in particular. Through logical methods such as division the researcher identifies observable properties of the thing under examination (e.g. the heart, or brain) that indicate its hidden cause or essence, i.e. function (e.g. being the centre of a particular psychic faculty) in the context of theory formation. The medical practitioner for his part will profit from dialectical method when it comes to establishing a diagnosis. The notion of indication or sign is pivotal in that its bridges the stage of discovery and that of confirmation through demonstrative proof. In addition, ‘dialectical’ serves as a label for plausible assumptions and arguments in cases where truth is unattainable or at any rate has not yet been established through demonstrative proof. Various influences are involved in Galen’s version of dialectic: Plato’s Phaedrus and Timaeus, Academic epistemology, Aristotle’s works on scientific method as well as input from the medical schools, most notably Rationalist ideas (e.g. indication). His resulting position can best be described as an original synthesis developed with a view to the interests of the medical theorist and practitioner.

The chapter considers why Plato thinks that Forms cannot be perceived by the senses. It argues that this is because Forms, or essences, cannot be defined by example and exemplar. It shows that this does not mean that Forms/essences can be known only a priori.