Rocco J. Gennaropersons and one body, at least at different times, especially if Locke is correct. His account of
personal identity through time famously appealed to consciousness and memory. On his view,
a later person (P2) is identical to an earlier person (P1), just in case P2’s consciousness “can be
extended backwards” to P1. This is taken to mean that P2 consciously remembers P1’s thoughts
and experiences, which is often called the “psychological continuity” account of personal iden-
tity. My personhood goes with my consciousness and memory, not necessarily with my body.
So, a case of DID would seem to be a case where a single body houses two distinct personalities,
often with profoundly different character traits and behavior patterns.
Further, Locke recognized that if there are really two different persons, then it is difficult to
make sense of holding one morally responsible for the other’s actions. Indeed, there is an often-cited
link between personal identity and moral responsibility (Kennett and Matthews 2002). One might
think of the well-known Dr. Jekyll and Mr. Hyde tale of two persons, one good and one evil, inhab-
iting the same body (from the Robert Louis Stevenson 1886 classic novel). It would seem to be
wrong to punish or blame one person for the actions of another person. We are justified in hold-
ing a person responsible for some past action only if the person is identical with the person who
performed that action. Locke argues that one is justifiably held accountable only for those actions
performed by a person to whom one’s present consciousness extends. This would apply not only
to a person whose alter had committed crimes but perhaps also to, say, an elderly inmate who has
lost virtually all memory of committing a crime due to long-term aging.
It should be noted that DID has been, at times, a very controversial diagnosis. When it was
called multiple personality disorder (MPD), there was a problem of over-diagnosis especially
in the 1980s, perhaps stemming from the publication of the book Sybil which, along with the
subsequent film, had a major impact on the popular culture of the time. Even in the psychiatric
community, there has been a great deal of disagreement about DID. Some argue that DID (and
MPD previously) does not really exist at all and point to cases of irresponsible therapists who
encouraged their patients to believe (falsely) that they had been abused as children or who were
accused of implanting such memories in patients via hypnosis. Significant controversy still sur-
rounds the diagnosis of DID but it remains as a category in the Diagnostic and Statistical Manual
of Mental Disorders, the DSM-5 (American Psychiatric Association, 2013). Most today hold that
DID results from repeated childhood abuse where dissociating is a way to cope with traumatic
experiences.
As we saw above, the link between personal identity and memory is a close one, at least
according to one prominent theory. So, what about cases where we clearly have a single per-
son but her consciousness is negatively affected by severe memory loss (amnesia)? Anterograde
amnesia is the loss of short-term memory or impairment of the ability to form new memories
through memorization. Retrograde amnesia is the loss of pre-existing memories to conscious
recollection, well beyond a normal degree of forgetfulness. The person may be able to memorize
new things that occur after the onset of amnesia (unlike in anterograde amnesia), but be unable
to recall some or all of one’s life prior to the onset.
To take one example, Sacks’ (1987) patient Jimmie G. didn’t even recognize himself in the
mirror because he thought he was 20 years younger, having no “episodic memory” of those
intervening years. He didn’t recognize the doctor each time he came in to see him. Jimmie G.
had Korsakoff ’s syndrome due to long-term heavy drinking. Nonetheless, he retained much of
his “procedural memory,” that is, memory for how to do certain things or display various skills.
This is an example of a “dissociation” between two cognitive abilities, that is, a case where one
cognitive function, A, is preserved but another, B, is damaged. In Jimmie G. and other similar
cases, procedural memory can not only remain undamaged (such as typing or riding a bicycle),
but the patient can also learn new skills (such as following a moving target with a pointer)