Hume denies that there is anything that could explain how persons remain the same throughout their lives. His chief question, accordingly, is why we believe in personal identity over time.
The way he explains the belief draws on an analogy with his explanation of why we think that other things, especially plants and animals, remain the same despite changes that happen over time.
I intend to focus on some challenges that this analogy raises.
The chapter falls into four parts.
Hume identifies his target. The target of this chapter is the view that we are every moment intimately conscious of what we call our self and certain of its identity and simplicity (Hume [1739] 2019, 571–72).
Hume’s bundle theory of the mind. Hume argues that there is nothing that is identical over time nor simple in the human mind. The mind contains a variety of perceptions (that is, impressions and ideas) that constantly change. Moreover, there is nothing to the mind other than its perceptions. The mind is like a bundle of perceptions or a theater where the perceptions are viewed (Hume [1739] 2019, 572).
Consequently, the question is not whether there is a simple self that is identical over time. It is why we believe that this is so (Hume [1739] 2019, 572R).
Hume’s strategy is to look at why we believe that plants and animals remain the same over time. Hume’s theory is that we conflate an unchanging series of perceptions with a series that changes. So the question is what the changing series has to be like to cause us to conflate it with the unchanging one (Hume [1739] 2019, 572R–575L). I will explain this at greater length below.
The explanation of why we believe in personal identity and the self. The relations among perceptions that lead us to conflate them with an unchanging series are resemblance and causation. In a nod to Locke, Hume argues that memory is relevant to both: memories resemble the original perceptions that are remembered and we discover causal relations by examining the correlations among perceptions in the past (Hume [1739] 2019, 575–76).
Conclusion. Hume revisits the points about identity and simplicity (Hume [1739] 2019, 576–77).
Hume’s strategy is to look at why we attribute identity to plants and animals. He will contend that we do something similar when we attribute identity to ourselves.
The idea is that we conflate two different series of perceptions with one another.
Let’s call the first one the Invariable series (I) and the second one the Diverse series (D). I will use letters to represent perceptions, which is Hume’s generic term for impressions and ideas.
(I) A A A A A A A A
(D) A A A A A A a A
Hume’s theory is that sometimes a series of perceptions like (D) can have features that makes it feel to us as though it were series (I). When that is so, we think that object that was perceived remained the same over time, as it seems to have done in (I), even though it is obvious that this is not true.
For example, he says that we ignore differences when they are small relative to the size of the object. We think that taking away a half cup of sand destroys an anthill, but not a mountain. And we think this even though, strictly speaking, it also produces a different mountain.
Some other features of a (D) series that is likely to be conflated with an (I) series include:
Hume has some other terrific examples to chew on. We think that a sound can be intermittent while remaining the same sound and that a ruin can be the same building as a church that was whole in the past. Finally, some kinds of things can remain the same despite rapid changes in their parts, such as a river.
When he turns to personal identity, he will try to show that the series of perceptions in our heads is a (D) series that is conflated with an (I) series because the perceptions resemble one another and because there are causal relations among them.
I am going to ask why we would draw a conclusion about ourselves from having perceptions that are related to one another in that way. I’m going to leave that hanging there for the sake of suspense.
The editors have included a section of an Appendix to the Treatise of Human Nature in which Hume expresses doubts about his theory of personal identity (Hume [1739] 2019, 577–78).
Unfortunately, it is hard to understand exactly what he thought was wrong with it. He says that he cannot resolve a conflict between two propositions (Hume [1739] 2019, 578):
Since those appear to be perfectly consistent with one another, it is hard to see what he thinks the defect in his theory is.