According to the Stoics, the psychology of adult human beings is unified in a striking sense: each of the soul's perceptive, discursive, and motivational functions belongs to the single faculty of reason. Reason, in turn, is constituted by a set of conceptions (ennoiai) and preconceptions (prolêpseis) acquired on the basis of experience. The few secure sources that bear on this theory in the early Stoa suggest that certain of these empirically acquired conceptions function, somehow, as a criterion of truth in human perception and rational cognition generally. The conceptions associated with this cognitive role are sometimes said to be shared or common (koinai).
These tenuous but intriguing claims are the focus of Henry Dyson's book, which surveys most of the surviving evidence for the old Stoic theory of concepts. Dyson's central aims are two: first, to offer a reconstruction of the early and orthodox Stoic theory, for which available evidence is slight; second, to establish this theory's relation to a body of later evidence, including that of Cicero and Epictetus, which has sometimes been regarded as contaminated by Platonist influence. Chapters 1 to 3 of Dyson's book are largely occupied with taxonomical concerns. Here Dyson argues, contra Sandbach and others, that the set of common conceptions as conceived by early Stoic theory is in fact identical with the set of preconceptions, which comprise a form of tacit knowledge shared by all humans qua rational. This conclusion appears to be in tension with Dyson's further suggestions that the Stoics describe as common conceptions only those preconceptions made articulate by dialectic, and that common conceptions and preconceptions are in fact functionally distinct. These latter claims amount to a plausible reading of the sources, but they tend to support the conclusion Dyson is concerned to refute: that common conceptions are, after all, better characterized as a proper subset of preconceptions.
Chapters 4 and 5 offer a more detailed account of the role of preconceptions in early Stoic theory. Dyson accepts the now standard view that the Stoic account of concept acquisition amounts to a form of dispositional innatism: we are providentially disposed by nature to acquire the conceptions in which reason consists. Dyson argues further that although preconceptions do not themselves underwrite the perception and discrimination of "natural kinds or sensible qualities" (108), they nonetheless support a range of subsequent inferences about the necessary features of natural kinds. Accordingly, when rendered explicit by dialectic, the knowledge encoded by preconceptions provides a basis for philosophical definition and demonstration. In Dyson's view, the resulting position may be fairly described as a form of psychological but not epistemological empiricism: though preconceptions are acquired solely though experience on the Stoic account, they provide epistemic warrant for a range of non-sensory cataleptic impressions. Finally, Dyson presents the distinction between psychological and epistemological empiricism as the key to understanding the relation of the early Stoic theory to later evidence. In his view, the alleged Platonism of later sources in fact reflects a rationalist element equally present in the older Stoa. Though this is an attractive suggestion, it appears for the most part to be a guiding supposition of Dyson's method rather than a conclusion for which he argues in [End Page 115] detail. While early fragments are interpreted in the light of later material, little attention is given to later accounts as such.
On the whole, Dyson's book advances discussion of the Stoic theory of concepts at a number of points and provides a helpful synthesis of much recent scholarship on the subject. Unfortunately, it is marred by an untidy presentation of its main conclusions and by a heavily layered manner of exposition. Dyson tends to address one topic or question by introducing new ones (section 5.2 is a case in point), and a number of central proposals appear in a rather unusual summary of "interim conclusions" interposed between chapters 3 and 4. Much of this material would no doubt have served readers better in...