In the 17th century Descartes set out for a new start in philosophy. Applying the method of systematic doubt he searched for a philosophical statement whose truth stands firm and cannot be questioned by further doubt. In his Discourse on the Method he came up with the cogito argument

cogito ergo sum = I think, therefore I am.

The current catalog of Diagnostic and Statistical Manual of Mental Disorders (DSM-5) lists Dissociative identity disorder (DID). The symptom of this kind of disorder is to show

at least two distinct and relatively enduring personality states.

In literature, R.L. Stevenson in the 19th century exemplified this type of mental disorder in his novella Strange Case of Dr Jekyll and Mr Hyde. (Disclaimer: Of course I want to dismiss any moral judgement about mental disorder.)

If a client experiences more than one personal identity then his/her different identities are oscillating. They arise and dissolve. Hence my question:

  • In case of DID, the existence of which ontological entity does the cogito-argument attempt to show?
  • Does DID invalidate the cogito-argument or can the two, the philosophical thesis and the psychological finding, be reconciled?

Added on request: The question is triggered by If we don't know anything for certain, how do we know that? and some comments in its section of answers.

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    scary. if either personality can perform the cogito then they exist. idt i see the problem with svereal personality states inhabiting one mental series or continuity. ofc it would be remarkable if they truly were two separate series of events, sometimes disappearing and forking etc.. but there's a reason why they call it madness
    – user67675
    Nov 2, 2023 at 6:55
  • 1
    no-one believes that ppl suffering thought broadcasting literally have all their thoughts beamed out of them and seen by everyone else. perhaps they have leaky body language and people gossip. it's not the same thing
    – user67675
    Nov 2, 2023 at 6:59
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    Why would DID matter? The I, when uttered, refers to just one of the personalities, the one that does the uttering. And if cogito is fixed by removing the I, as Descartes's critics suggested, (as in "thinking is occurring, therefore, something is") DID is consistent with that as well. Even if the two are mixed and we get a non-sequitur, "thinking is occurring, therefore, I am", the problem is not with DID.
    – Conifold
    Nov 2, 2023 at 7:09
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    @Conifold Upholding the cogito-argument also for a person with DID implies that the ego oscillates. Then occurs a permanent vanishing and reappearing of several egos. – Of course one can fix the cogito-argument by changing it to the version you propose last. But that removes much of the characteristics of the philosophical argument, which attempts to conclude from introspection to ontology.
    – Jo Wehler
    Nov 2, 2023 at 7:29
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    Please add to the question the prompt for why it occurred to you and what you think the results will be?
    – Scott Rowe
    Nov 2, 2023 at 11:00

4 Answers 4


"I think, therefore I am" does not imply that the person thinking and being is eternal. It just says that one cannot doubt one's own existence, as and when one thinks about it. If and when Dr Jekyll thinks, then Dr Jekyll is, and if and when Mr Hyde thinks, then Mr Hyde is.

  • 2
    Yes, similar statements are made about sleep.
    – Scott Rowe
    Nov 2, 2023 at 10:58
  • A lovely answer- I could not have put it better myself (he says, enviously)! +1 Nov 2, 2023 at 21:34
  • @MarcoOcram Thank you!
    – Olivier5
    Nov 2, 2023 at 21:55
  • @Olivier5 It looks as if you consider the temporary identities of Dr Jekyll and Mr Hyde to be two real persons, who think and who exist as real humans. The result would be a permanent birth and death of two real persons. But there is only one real person who experiences different alters.
    – Jo Wehler
    Nov 3, 2023 at 20:01
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    @MauroALLEGRANZA And they would be quite right to do so.
    – Olivier5
    Nov 4, 2023 at 13:01

It depends on which chapter of the Meditations we are focused on. What’s in the second meditation is merely that one cannot doubt their own existence, since the very doubting necessitates that one exists. In this regard, even a person with DID won’t affect this, since as each alter fronts and performs Cartesian Doubt, that alter finds that they exist.

The problem of course is with what Descartes does later, in (1) assuming that introspection is in a sense infallible and (2) the ego in the cogito is absolutely simple (3) Thus, the cogito is the activity of a simple thinking substance (res cogitans). DID destroys both premises pretty clearly.

The whole function of disassociation and repression from and of traumatic experiences is to protect the psyche. The cost is a defect not only to one’s introspective access to prior mental states, but confusion on how to interpret current mental states. The very mechanisms involved in DID show (1) is simply false.

It gets worse though. DID also exhibits that the self is no simple entity, contrary to the Cartesian claim (psychologically). DID happens not because one has a traumatic experience that “cracks the ego” but rather because young children (before the age of six) don’t have a fully formed ego to begin with. As they disassociate under extreme trauma, the lack of a fully formed self means these fragments of experience can acquire their own egos (and become alters). It’s why DID is not professionally diagnosed for traumas outside of early childhood and for one time traumas.

What is taken to be simple in the cogito by Descartes, the "I" or "ego," is not only not a simple unity, but its emergence within all humans (not just those with DID) is a complex process of various ego-states fully integrating and fusing since infancy. The Structural Theory of Dissociation while not uncontested, is becoming quite standard in the literature on not just DID, but PTSD as well. The "ego" of even each alter in DID is no simple unity, but is the amalgamation of countless ego-states in the first six to eight years of life. There is no psychological simplicity of the ego, period. The Cartesian argument flows from unsubstantiated premises (1) introspection is infallible and (2) the I or ego is simple (which the lack of simplicity of the cogito flows from the lack of simplicity of the ego).

Without support from premises (1) and (2), Descartes cannot infer his conclusion, whether in regard to a singlet having one res cogitans wedded to their body, or a system having multiple res cogitans wedded to their body. DID removes all support from (1) and (2).

The “ego cogito” is no simple thing. It not only is absent at birth, but doesn’t fully congeal until about six to eight years old. So, Cartesian speculation about the soul is unwarranted (though not falsified) by what we understand of DID today.

  • 1
    I like this answer a lot because it shows actual knowledge about what Descartes wrote and the structure of his argument but...what exactly prevents two thinking substances (both being "simple" since simplicity is the argument for being a substance, nothing else, if I understand the Meditations correctly) residing in/with one material substance? Descartes' arguments rely on the infallibility of a thinking substance reflecting on itself, not more and not less, isn't it? Hence, how exactly are 1) and 2) necessarily contradicted through DID?
    – Philip Klöcking
    Nov 4, 2023 at 20:23
  • It could also be that DID people suffer from illogical, irrational metaphysics, having not read Descartes or having failed to understand him. So they are the exception that confirms the cogito rule.
    – Olivier5
    Nov 5, 2023 at 8:03
  • @PhilipKlöcking I've now edited my answer to better suit your concerns. I've removed the bolder claim that Cartesian metaphysics is falsified, and added detail to explain how DID (understood in its details) undercuts the premises Descartes used to infer his metaphysics of the soul.
    – Hokon
    Nov 6, 2023 at 23:36

At this moment I think therefore I exist. At this moment I think therefore I exist. In some way these are different cogitos, and I see no reason to believe a separate personality structure makes any difference, not morally, not ontologically, not metaphysically. I am very different in my dreams, but in some sense it's my dreaming them.

Sorry if that sounds harsh, but it's not something that a sufferer should be told, anymore than we should tell certain delusional people "yes, you are dead" (let alone conclude we should read their will etc.). There's some metaphorical truth to it, as in non-primary delusions, but given that alters will have confabulated back stories etc., we're not just talking about just another way of talking about selfhood, but a mental illness etc.. Anyone who claims that a mental illness should not be treated, is just how they are, and it's offensive to suggest that they are not many people,is an exploiter.

It's OK to think you're several people, and I don't find it offensive at all, but if it's having a negative impact on your life, then you should probably get it treated (which presumbaly involves integrating those "persons").

I don't know what the OP was really getting at: alters are not equivalent to other persons in the normal sense in which you and I and Fred and Jane are. The claim otherwise is at best a mental illness and at worst an act of psychological abuse.


What about the unconscious and the subconscious? Remember the subconscious is the filter but really who is behind the wheel is our “life long recorder,” our unconscious always recording everything in the background; and is prevalent when saying the first thought that comes to mind (before the subconscious filters it), in our dreams and when our adrenaline activates our fight or flight scanning our life recording for information to use when in fight or flight mode to survive. Aside from the theories talked about here, I can’t help but feel that this dynamic some how influences the topic but yet not too many take this into consideration just because not that that much research has really been or can’t by subjective means be done on it. As opposed to find some way to challenge my side influence it would be more interesting and intriguing and less debateful if anyone in this post could show a connection to the mentioned topic and my side influence regarding consciousness states. I’m curious to know your responses (if there will be any).

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    Nov 5, 2023 at 14:49

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