Why did the problem of personal identity cause Hume such severe difficulties that in the end he admitted that he could see no way of providing a coherent account?

Introduction

The primary source material for any discussion of Hume's "theory"of personal identity is Book I, Part IV, Section VI -- "Of Personal Identity" -- of his A Treatise of Human Nature.   However, to fully understand the material of this section, one must also take into consideration the context of what precedes that section in the earlier parts of the Treatise.   And there are two key considerations that need to be highlighted in order to make clear what Hume's problem was with personal identity.

The first of these considerations is the view that Hume had of the relationship between "impressions"and "ideas".   Hume's Treatise was an exploration of the psychology of the human mind.   And as such, he was most centrally concerned with the contents of the Mind.

"Here therefore we may divide all the perceptions of the mind into two classes or species, which are distinguished by their different degrees of force and vivacity. The less forcible and lively are commonly denominated Thoughts or Ideas. The other species want a name in our language, and in most others; I suppose, because it was not requisite for any, but philosophical purposes, to rank them under a general term or appellation. Let us, therefore, use a little freedom, and call them Impressions; employing that word in a sense somewhat different from the usual. By the term impression, then, I mean all our more lively perceptions, when we hear, or see, or feel, or love, or hate, or desire, or will. And impressions are distinguished from ideas, which are the less lively perceptions, of which we are conscious, when we reflect on any of those sensations or movements above mentioned."[EHU, II]

It was Hume's thesis that ideas are fundamentally dependent on impressions (more specifically, sensory perceptions).   Impressions are the "given" -- the lively and forceful and uncontrollable inputs from the senses, the memory, and the imagination.   Ideas are the vaguer "images"of those impressions.   It is the fact that all of Hume's mental existents are based on the "givens"of perceptions, by the way, that makes him one of the "British Empiricists".   Hume used the term "copy"to signify the relationship between the impressions and the ideas that are dependent on them.   Complex ideas are equally based on impressions, although at one step removed.

"It seems a proposition, which will not admit of much dispute, that all our ideas are nothing but copies of our impressions, or, in other words, that it is impossible for us to think of anything, which we have not antecedently felt, either by our external or internal senses . . . Complex ideas, may, perhaps, be well known by definition, which is nothing but an enumeration of those parts or simple ideas, that compose them."[EHU VII.1]

"Or, to express myself in philosophical language, all our ideas or more feeble perceptions are copies of our impressions or more lively ones."[EHU, II]

The significance of this view of the relationship between impressions and ideas to our current discussion is that it allows Hume to claim that if an idea is a valid and proper one, then one must be able to locate the impressions on which it is based.   If no such impressions can be found, then the idea at issue is but an arbitrary construct of our imagination -- fictitious -- with nothing really existent on which it is based.

The second of the introductory considerations is Hume's notion of "Identity".   Hume's notion is quite unlike that of his predecessor Locke, who in his Essay Concerning Human Understanding described three different notions of "identity"for inanimate objects, living things, and the Human Mind.   It is also quite unlike any modern notion of "identity"that recognizes identity as "kind (or "sortal")-relative".   Hume's notion of identity is, by comparison, quite absolute.   On the other hand, Hume maintains that "identity"is not a "real"idea but a fiction of the imagination, because he can find no impression from which it can be a "copy".

"First, as to the principle of individuation; we may observe, that the view of any one object is not sufficient to convey the idea of identity. For in that proposition, an object is the same with itself, if the idea express'd by the word, object, were no ways distinguish'd from that meant by itself; we really shou'd mean nothing, nor wou'd the proposition contain a predicate and a subject, which however are imply'd in this affirmation. One single object conveys the idea of unity, not that of identity.   . . .   I have already observ'd , that time, in a strict sense, implies succession, and that when we apply its idea to any unchangeable object, 'tis only by a fiction of the imagination, by which the unchangeable object is suppos'd to participate of the changes of the co-existent objects, and in particular of that of our perceptions. This fiction of the imagination almost universally takes place; and 'tis by means of it, that a single object, plac'd before us, and survey'd for any time without our discovering in it any interruption or variation, is able to give us a notion of identity.   "[Treatise, I, IV, II -- Of Scepticism with Regard to the Senses]

Hume's reasoning is based on his fundamental premise that all that is given to us is a flow of individuated perceptions -- impressions.   The idea of "unity"is then based on but a single and distinct sensory impressions of a solitary lump of something that does not change.   Hence the idea of "unity"is based on the perceived invariableness and uninterruptedness of the perception.   (Not, be it noted, the invariableness or uninterruptedness of whatever (if anything) being perceived.)

"'Tis still true, that every distinct perception, which enters into the composition of the mind, is a distinct existence, and is different, and distinguishable, and separable from every other perception,"[Treatise, I, IV, VI]

"Thus the principle of individuation is nothing but the invariableness and uninterruptedness of any object, thro'a suppos'd variation of time, by which the mind can trace it in the different periods of its existence, without any break of the view, and without being oblig'd to form the idea of multiplicity or number."[Treatise, I, IV, VI]

The idea of "identity,"on the other hand, is a fiction based on the mind's mistaken extension of he idea of "unity"across the distinctly separate impressions we experience over time.

"We have a distinct idea of an object, that remains invariable and uninterrupted thro' a suppos'd variation of time; and this idea we call that of identity or sameness. We have also a distinct idea of several different objects existing in succession, and connected together by a close relation; and this to an accurate view affords as perfect a notion of diversity, as if there was no manner of relation among the objects."[Treatise, I, IV, VI]

The idea of "identity"is created by the imagination based on the relationship of a temporal sequence of resembling impressions of some unitary object.   It is the "easy passage of thought"from one member of this sequence of impressions to the temporally next member of the sequence that causes us to mistake the sequence of distinct impressions for a single impression.   The idea of "identity"is then founded on this mistake -- it is an idea of a multitude of related impressions that is improperly copied from a single impression of a unitary object.   The consequence of the notion of "identity"being the result of an error of the mind, is that Hume can claim that the idea of "personal identity"is also a fictitious one.   There is nothing in reality, according to his philosophy, that can properly be called the "self".   All that there is available, is a "collection of different perceptions, which succeed each other with an inconceivable rapidity, and are in a perpetual flux and movement."

"'Tis confest by the most judicious philosophers, that our ideas of bodies are nothing but collections form'd by the mind of the ideas of the several distinct sensible qualities, of which objects are compos'd, and which we find to have a constant union with each other. But however these qualities may in themselves be entirely distinct, 'tis certain we commonly regard the compound, which they form, as One thing, and as continuing the Same under very considerable alterations. The acknowledg'd composition is evidently contrary to this suppos'd simplicity and the variation to the identity."[Treatise, I, IV, III]

"It must be some one impression, that gives rise to every real idea. But self or person is not any one impression, but that to which our several impressions and ideas are suppos'd to have a reference. If any impression gives rise to the idea of self, that impression must continue invariably the same, thro' the whole course of our lives; since self is suppos'd to exist after that manner.   . . .   Pain and pleasure, grief and joy, passions and sensations succeed each other, and never all exist at the same time."[Treatise, I, IV, VI]

"But there is no impression constant and invariable.   . . .   It cannot, therefore, be from any of these impressions, or from any other, that the idea of self is deriv'd; and consequently there is no such idea."[Treatise, I, IV, VI]

So Hume did not have a "theory of personal identity"as such.   He very clearly claims that there is in fact so such perceptible thing in reality as "identity", let alone "personal identity".   "Personal Identity"he claims is as fictitious a construct of the imagination as is "identity"itself.   Unlike the ideas of Locke or the modern kind-relative concept of identity, Hume's idea of identity is the same idea across all objects to which it is applied.   Hume is very specifically denying that "identity"comes in different "flavours"according as to what it is being applied to.   In particular, he maintains that the idea of "identity"applies equally to living things and the "self".

"the understanding never observes any real connexion among objects, and that even the union of cause and effect, when strictly examin'd, resolves itself into a customary association of ideas. For from thence it evidently follows, that identity is nothing really belonging to these different perceptions, and uniting them together; but is merely a quality, which we attribute to them, because of the union of their ideas in the imagination, when we reflect upon them."[Treatise, I, IV, VI]

"The identity, which we ascribe to the mind of man, is only a fictitious one, and of a like kind with that which we ascribe to vegetables and animal bodies. It cannot, therefore, have a different origin, but must proceed from a like operation of the imagination upon like objects."[Treatise, I, IV, VI]

"what must become of all our particular perceptions upon this hypothesis? All these are different, and distinguishable, and separable from each other, and may be separately consider'd, and may exist separately, and have no need of any thing to support their existence.   . . .   I may venture to affirm of the rest of mankind, that they are nothing but a bundle or collection of different perceptions, which succeed each other with an inconceivable rapidity, and are in a perpetual flux and movement.   . . .   There is properly no simplicity in it at one time, nor identity in different; whatever natural propension we may have to imagine that simplicity and identity.   . . .   They are the successive perceptions only, that constitute the mind; nor have we the most distant notion of the place, where these scenes are represented, or of the materials, of which it is compos'd."[Treatise, I, IV, VI]

Hume's Explanation of the Mistake

Therefore, what is commonly taken to be Hume's "theory of personal identity"is in fact a theory of how it is that we make the mistake of thinking that we have the idea of personal identity.  

"What then gives us so great a propension to ascribe an identity to these successive perceptions, and to suppose ourselves possest of an invariable and uninterrupted existence thro' the whole course of our lives?"[Treatise, I, IV, VI]

As with his explanation of the mind's identification of the causal relationship, Hume places the source of this mistake clearly on an innate propensity of the mind to follow a "smooth and uninterrupted progress of the thought along a train of connected ideas."It is just a natural consequence of the way that we are, according to Hume, that we make the mistake of following the easy path of thought without proper reflection on the error involved.

  "We have a distinct idea of an object, that remains invariable and uninterrupted thro' a suppos'd variation of time; and this idea we call that of identity or sameness. We have also a distinct idea of several different objects existing in succession, and connected together by a close relation; and this to an accurate view affords as perfect a notion of diversity, as if there were no manner of relation among the objects. But tho' these two ideas ... be in themselves perfectly distinct, and even contrary, yet "Tis certain, that in our common way of thinking they are generally confounded with each other. That action of the imagination, by which we consider the uninterrupted and invariable object, and that by which we reflect on the succession of related objects, are almost the same to the feeling . .. This resemblance is the cause of the confusion and mistake, and makes us substitute the notion of identity, instead of that of related objects"[Treatise, I, IV, VI.]

"'Tis, therefore, on some of these three relations of resemblance, contiguity and causation, that identity depends; and as the very essence of these relations consists in their producing an easy transition of ideas; it follows, that our notions of personal identity, proceed entirely from the smooth and uninterrupted progress of the thought along a train of connected ideas, according to the principles above-explain'd."[Treatise, I, IV, VI]

"That action of the imagination, by which we consider the uninterrupted and invariable object, and that by which we reflect on the succession of related objects, are almost the same to the feeling, nor is there much more effort of thought requir'd in the latter case than in the former. The relation facilitates the transition of the mind from one object to another, and renders its passage as smooth as if it contemplated one continu'd object. This resemblance is the cause of the confusion and mistake, and makes us substitute the notion of identity, instead of that of related objects."[Treatise, I, IV, VI]

"the relation of parts, which leads us into this mistake, is really nothing but a quality, which produces an association of ideas, and an easy transition of the imagination from one to another, it can only be from the resemblance,   . . .   The passage of the thought from the object before the change to the object after it, is so smooth and easy, that we scarce perceive the transition, and are apt to imagine, that "Tis nothing but a continu'd survey of the same object."[Treatise, I, IV, VI]

Hume's explanation of the notion of "personal identity"is that it rests on "an easy transition of the imagination"from one particular and distinct impression to the next.   In other words, the idea of "identity"in general, and "personal identity"or "self"in particular, consists of a similarity (resemblance) relationship between the particular and distinct impressions.   And it is the imagination that constructs the notion of identity out of this similarity and the resulting "easy passage of thought"from one instance of the similarity to another.

Hume's Problem

In the Appendix to the Treatise on Human Nature, Hume provides a startlingly honest admission that he finds some fundamental problem with his analysis of the idea of personal identity.

"But upon a more strict review of the section concerning personal identity, I find myself involv'd in such a labyrinth, that, I must confess, I neither know how to correct my former opinions, nor how to render them consistent."[Treatise, Appendix]

"all my hopes vanish, when I come to explain the principles, that unite our successive perceptions in our thought or consciousness. I cannot discover any theory, which gives me satisfaction on this head. In short there are two principles, which I cannot render consistent; nor is it in my power to renounce either of them, viz. that all our distinct perceptions are distinct existences, and that the mind never perceives any real connexion among distinct existences. Did our perceptions either inhere in something simple and individual, or did the mind perceive some real connexion among them, there wou'd be no difficulty in the case."[Treatise, Appendix]

The question that has raised some debate amongst commentators on Hume, is just what was the problem that he detected?   But nowhere is Hume very clear on just exactly what difficulty he saw.   The only clues we have are from the contents of the addendum to "Of Personal Identity"he inserted in the Appendix to the Treatise.   In those few short paragraphs, Hume addresses his reservations specifically to the idea of "personal identity", not to his idea of "identity"in general, or to the idea of an existing external object as the basis of perceptions.   So whatever difficulty we identify on Hume's behalf, it must be restricted to that small domain, and not infect any of the rest of Hume's philosophy.

There is one candidate that is obvious from his description of why we make the mistake of creating the fictitious idea of personal identity, and that appears to be well supported by Hume's comments in the Appendix(*).   Unlike some of the suggestions in the literature, this candidate has the advantage of being specific to Hume's description of how we create the fiction of "Personal Identity", and does not appear to threaten any other element of his philosophy.

Assuming, as Hume claims, that

(i)   each perception is individual and distinct; and

(ii)  the mind can never perceive any real connection between any of these distinct perceptions; and

(iii) it is the relationship of resemblance between similar perceptions that causes the mind to mistakenly adopt the fictitious idea of identity;

then a key consequences follows --

IF the mind is going to notice the resemblance of some set of perceptions that it can use to pick out (i.e. "identify"or grant "identity"to) some particular object,

THEN it cannot at the same time

(a) notice the universal resemblance amongst all of the perceptions that must be used as the basis of the idea of personal identity, and

(b) fail to notice the distinctness between the perceptions being used to pick out multiple objects.

In other words, Hume cannot have his cake and eat it too.   And his comments in the Appendix reflect his acknowledgement of this inconsistency.  

If the "passage of the thought from the object before the change to the object after it, is so smooth and easy, that we scarce perceive the transition, and are apt to imagine, that "Tis nothing but a continu'd survey of the same object"constitutes the necessary and sufficient condition for the identity of the particular object in question, then it must be the case that to notice distinct objects, the passage of thought cannot be so smooth and easy that we scare perceive the transition.   In other words, in order to recognize the identity of distinct objects, there must be some degree of discontinuity of the transition from one object to the next.   Obviously we do indeed, even if mistakenly, assign identities to a multitude of distinct objects.   So there must be a multitude of such discontinuities of the resemblance-relation in the "collection of different perceptions, which succeed each other with an inconceivable rapidity, and are in a perpetual flux and movement."

But if that is the case, when we consider the idea of personal identity -- the "self" -- the flow of perceptions is no longer "so smooth and easy, that we scarce perceive the transition, and are apt to imagine, that "Tis nothing but a continu'd survey of the same object."For Hume, the succession of resembling impressions that constitutes the basis for the idea of the identity of objects, and the succession of resembling impressions that constitutes the basis for the idea of the identity of the self is one and the same flow of successive impressions.   The idea of identity for objects does not depend on the objects identified.   It depends on the relation of resemblance that persists within the flow of impressions (perceptions).   Therefore the tendency of mind (the innate propensity) to group segments of successive resembling impressions into identities for particular objects is in direct conflict with the tendency of mind to collect the entire succession of impressions under a different relation of resemblance in order to provide the basis of the identity of the "self".

Unfortunately for Hume, given his "absolutist"approach to the idea of "identity", he has no way out of the contradiction.  

Notes & References

(*)   Roth, Abraham Sesshu;   What was Hume's Problem with Personal Identity, in Philosophy and Phenomenological Research, Vol 61, No 1 (Jul 2000), Pgs 91-114.

Hume, David;   A Treatise of Human Nature, 1739.   Online Library of Liberty, URL = <http://oll.libertyfund.org/EBooks/Hume_0213.pdf>    
(Citations given in "Treatise, Book, Part, Section"format)

Hume, David;   An Enquiry Concerning Human Understanding, 1748.   Eighteenth-Century Studies, URL=<http://18th.eserver.org/hume-enquiry.html#5>   (Citations given in "EUH, Section, Part"format.)

Mendus, Susan; Personal Identity: The Two Analogies in Hume, in The Philosophical Quarterly, Vol 30, No 118 (Jan 1980), Pgs 61-68.

Penelhum, Terence;   Hume on Personal Identity, in The Philosophical Review, Vol 64, No 4 (Oct 1955), Pgs 571-589.

Wolfram, Sybil;   Hume on Personal Identity, in Mind, New Series, Vol 83, No. 232 (Oct 1974). Pgs 586-593.

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