Constancy, Coherence, and Causality

Abstract

According to David Hume, we believe in the existence of an external world because of the phenomena of constancy and coherence (T 1.4.2.18-43; SBN 194-210).1 Hume delineated these two aspects of our sensory experience, and claimed that they influence the imagination in such a way as to generate belief in the existence of unperceived objects, independent of the mind. There is disagreement among philosophers as to the proper way to characterize constancy and coherence and the ways in which they respectively lead us to believe in external objects. The most straightforward account of coherence seems to make it the same as Hume's concept of causality. Yet Hume denies that they are the same ; and commentators disagree as to how to understand Hume's distinction between the two. As to constancy, many philosophers, following H.H. Price,2 have argued that it is really just a special case of coherence, and that its role in generating belief in external objects is the same as that of coherence. There is consequent puzzlement as to why Hume attributed a different, and more important, role to constancy. In an important recent book, Louis E. Loeb has proposed a very interesting two-part solution to this last puzzle: First, Hume is convinced that the commonsense view of the external world is patently false; so he cannot allow that it results from something as reasonable as causal inference. But the way coherence leads to belief in external objects is a form of causal inference. So, Hume wants constancy to lead to belief in external objects in a different, less reasonable way; and he wants constancy to be more important than coherence in leading to belief in the external world. Second, Hume did not realize that constancy is simply a special case of coherence (or causality), because of certain assumptions about causality that he