Four-Dimensionalism and Identity Across Time: Henry of Ghent vs. Bonaventure

Modern accounts of the identity of an object across time tend to fall roughly into two basic types.

Let us say that something persists ıff, somehow or other, it exists at various times; this is the neutral word. Something perdures iff it persists by having different temporal parts, or stages, at different times, though no one part of it is wholly present at more than one time; whereas it endures iff it persists by being wholly present at more than one time.¹

On the first of these analyses, but not the second, a persistent object includes temporal parts, and persistent material substances are thus on this analysis four-dimensional objects. On the first analysis, but not on the second, time-indices are explicable in terms of temporal parts. I shall take this as the basic difference between the two views.²

Both views allow us to talk of temporal order and temporal direction. I shall follow the medievals and talk about succession to capture both of these claims: “The notion of succession formally consists in the notion of ‘before’ and ‘after,’”³ and I take it that talk of “before” and “after” entails both order and direction.⁴ Medieval accounts of identity across time are, however, in some ways more obscure than the two modern ones just described. The medievals tend to associate all temporal succession with temporal extension: an object x has temporal succession if and only if x is temporally extended (and thus includes temporal parts).⁵ The medievals also (correctly, I believe) tend to suppose that substances—unlike (say) processes—do not have temporal parts.⁶ Thus, by the end of the thirteenth century the schoolmen commonly distinguish permanent objects from successive objects. Permanent objects do not, and successive objects do, have temporal parts.⁷ Successive objects are processes; the class of permanent objects was generally held to include not only God, but also angels, material substances, and static accidents (i.e., accidents which do not entail any sort of process).⁸

Given these claims, there is a problem to be faced in giving an account of the identity of, say, a material substance across time. But it is not the problem we would face. Our problem, post-Hume, is to explain the relevant (explanatory) continuity requirements for identity across time. The medievals’ problem is rather different. They do not need to specify general continuity requirements, because for them there is a very real sense in which substances—and indeed all permanent objects—lack temporal extension and, therefore, any sort of temporal succession at all. The problem for the medievals is thus to explain how substances can persist at all, rather than exist merely fleetingly.⁹

In what follows, I will examine two thirteenth-century solutions to this problem. The first is a radical one: simply to deny the standard claim that substances lack temporal parts. This view will thus bear some resemblance to what we would label four-dimensionalism. The second solution I will examine represents one of the more successful attempts to defend the standard thirteenth-century line that substances lack temporal parts. This solution goes some way towards allowing succession (and time-indexing) without appealing to temporal parts.

To understand the debate, we need to keep in mind that the medievals—following Aristotle—talk of the temporality of an object in terms of its being measured by time. Confusingly, they might speak of a substance’s being measured in one sort of way, and its accidental properties’ being measured in another. Thus, by the end of the thirteenth century it was often claimed that the duration of a substance is measured not by time but by the aevum—the durational measure of objects lacking temporal parts—while some of a substance’s accidental properties could be measured by time. (I will explain in detail below how time and the aevum were understood.) On this view, the temporality of an object’s properties is not sufficient cause for positing the temporality of the object. There are acceptable and unacceptable ways of understanding...