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Personal Identity

Dicephalus

(Work In Progress: output at 27/12/2025 17:57:07)

Notes

• Thesis Text:
  1. This Note is more general that Dicephalus, but I can’t think of a better title. There are several related pathological conditions in which the number of animals¹ and the number of individuals² may fail to align. In order of increasing severity of the “condition”:-
     1. Conjoined Twins: "Wikipedia - Conjoined Twins". Varies from a fairly loose conjunction of individuals, who may be separated, to more severe forms with shared organs and limbs. This article is of interest not least because it discusses the issue of fission³ versus fusion⁴, with the latter now the favourite suggestion.
     2. Craniopagus: "Wikipedia - Craniopagus Twins". The babies are joined at the skull. Contrast the case where both twins are persons⁵ with the degenerate case "Wikipedia - Craniopagus Parasiticus".
     3. Polycephaly: "Wikipedia - Polycephaly". Animals or humans with apparently more than one head. Dicephaly is the limiting case. Usually an extreme version of conjunction. See, for example, BBC - Rare two-headed snake. Note that the description “two headed” seems more plausible in the case of reptiles than humans, given their presumed reduced mental experience. That said, given that dicephaly is – according to this article – a case of incomplete fission⁶ of an original single individual – this descriptive decision may be correct.
     4. Diprosopus: "Wikipedia - Diprosopus".
        1. Animals, including humans, with two faces. There may be one or two brains, but only one skull. The important thing – and what differentiates the diprosopus from the dicephalus – is that there is much less duplication of body parts, so it is very much two brains sharing one body – though it can also be the relatively superficial – though tragic – case of one brain, one body and two faces.
        2. For more on diprosopus₇ see:-
           • Guardian: Diprosopus (Sydney, 2014),
           • MedicalXpress: Diprosopus (Sydney, 2014),
           • Gulf News: Diprosopus (Sydney, 2014) and
           • Mail Online: Diprosopus (Sydney, 2014); also
           • Mail Online: Janus Cat.
             
        3. The Mail Online diprosopus article above quotes the following “Medical Facts” from the Embryo Project Encyclopedia:
           • The rare condition diprosopus is also known as craniofacial duplication.
           • Diprosopus refers to a baby born with a single torso, normal limbs and facial features, which are duplicated⁸ to a degree.
           • In mild instances the baby may have a duplicated nose and the eyes may be spaced far apart. But in extreme cases the entire face can be replicated⁹, hence the name diprosopus - Greek for two-faced.
           • Most babies born with diprosopus are stillborn, and there are fewer than 50 cases documented since 1864.
           • Where a baby is born with two complete identical faces, the condition is considered a rare variant of conjoined twinning.
           • But while conjoined twinning is the result of an incomplete separation of two embryos, diprosopus is caused by abnormal activity of the protein Sonic Hedgehog (SHH).
           • The protein is responsible for signalling craniofacial patterning during embryonic development, and among other things governs the width of facial features.
           • Where the protein is found in excess, a baby will have wider facial features, and in extreme cases it can cause the duplication of those features.
           • Diprosopus can be detected via ultrasound in pregnancy, or via CT scanning.
           • One of the first indications of the condition is the detection of abnormally high amount of amniotic fluid present within the amniotic sac.
           • There is currently no treatment to cure the condition and because of its rarity few treatment options or corrective surgery techniques exist.
             
        4. This doesn’t seem to explain why there should be two brains, however. An actual search of the Encyclopedia delivers a much briefer and less informative article.
           
  2. All these situations – and in particular the paradigm case of dicephalus, wherein it appears that we have a human animal¹⁰ with two heads – are often cited as a prima facie problems for Animalism¹¹.
  3. The claim is that while we have multiple persons¹², we only have a single animal¹³, so the animalist¹⁴ claim that persons are numerically identical to their organisms¹⁵ is false.
  4. Of course, the above claim is somewhat bald. Animalism¹⁶ – at least in the form expounded by Eric Olson – has no truck with psychology¹⁷ as necessary for our persistence¹⁸. While Olson often refers to “people” this is just shorthand for “some of us¹⁹”, rather than “persons”, the plural of person²⁰.
  5. So, if the animalist claim is that we are human animals, then the question is – in the case of dicephalus and related conditions – how many of us²¹ are present, and how many human animals²² are present. Animalism²³ needs the numbers to be equal.
  6. That said, these cases add to the prima facie case for animalism – that we are human animals – as our animal nature is pointed out by the unfortunate errors that have arisen during the gestation of the animal.
  7. The cases under consideration have the advantage over standard Thought Experiments²⁴, in that they are real-life cases. What is actual must be possible, and a standard argument against TEs is thereby avoided, namely that some of them, at least – whatever our intuitions may say – may not in fact be possible²⁵ because the TEs are under-described.
  8. That said, there’s still a temptation to push the boundaries, and to consider cases in this general area that are not actual yet are not quite as science-fiction as the usual TEs²⁶.
  9. Cases of dicephalus are fairly rare, and often one “twin” is degenerate (contrast "Wikipedia - Craniopagus Parasiticus" with "Wikipedia - Dicephalic Parapagus Twins") so it’s not clear that we have two persons²⁷ or two of “us²⁸”. However, the case of the dicephalus twins²⁹, Abigail and Brittany Hensel, where one body appears to be coordinated by two brains (see "Wikipedia - Abby and Brittany Hensel") shows that this is not necessarily the case. Here we definitely have two persons³⁰, but maybe only one animal³¹. This is probably a real-life case of either incomplete fission³² or of fusion³³.
  10. There are two issues with these cases:-
      1. How should they be best described?
      2. How should the animalist respond?
  11. Description:
      1. I need to research this further, but it seems to me that we have a sliding scale of pathological cases – from that of Siamese twins (who are often fairly loosely connected, and can often be surgically separated) to the much more extreme cases of diprosopus.
      2. I presume, also, that we can have either fission³⁴ or fusion³⁵, or indeed both serially – as the case may be – an empirical matter.
      3. In the case of (incomplete) fission³⁶, the twins³⁷ will be identical, as the situation is one where the twinning process has not completed.
      4. But (I imagine) we can also have a case of fusion³⁸, where the twin-embryos³⁹ – whether identical or fraternal – that were separate have subsequently partially fused. Note that if the twins are identical, we will have had a case of fission⁴⁰ followed by fusion⁴¹. I don’t know whether there have been any actual cases of this.
  12. Response:
      1. The most likely response from the animalist⁴² is that the dicephalus is not a single organism⁴³, but two organisms that share some body-parts.
      2. The force of the arguments in this kind of case depends on “relevant similarity” – if the dicephalus-case is sufficiently similar to the general case, it can be used to show things about the general case. But this claim can be rejected.
      3. In the case of diprosopus, it is more difficult to claim that there are two organisms⁴⁴. Now – in practice – there are never two persons⁴⁵ either (as in all the 30-odd cases recorded since 1860, the condition has proved quickly fatal, so there were no persons⁴⁶ present). But this might not always be so. Also, Lynne Rudder Baker might claim that the rudimentary persons were indeed persons⁴⁷ – as there was a possibility that they would develop into robust persons. Failing that, it might be the case in the future there is a case where the diprosopus survives to develop personality⁴⁸. There does seem to have been an actual case of survival into adulthood, but there was only one person, and one disfigured individual. Finally, we have to decide how to describe the situation where there is one brain stem but two ‘brains⁴⁹’ (presumably two sets of cerebral hemispheres⁵⁰). There don’t seem to have been any actual cases of this in humans, but there might have been⁵¹.

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  Further Remarks:
  1. This Note ought really to be entitled ‘Conjoined Twins’, but it’s a pain to change things at this stage.

References

• Relevant Works cited above:
  1. "Wikipedia - Abby and Brittany Hensel", 2023, External Link, Read = 25%
  2. "Wikipedia - Conjoined Twins", 2023, External Link, Read = 10%
  3. "Wikipedia - Craniopagus Parasiticus", 2023, External Link, Read = 25%
  4. "Wikipedia - Craniopagus Twins", 2017, Annotations, External Link, Internal PDF Link, Read
  5. "Wikipedia - Dicephalic Parapagus Twins", 2023, External Link, Read = 25%
  6. "Wikipedia - Diprosopus", 2023, External Link, Read = 25%
  7. "Wikipedia - Polycephaly", 2023, External Link, Read = 10%
• For a page of Links⁵² to this Note, Click here.
• Works on this topic that I’ve actually read⁵³, include the following:-
  • Dicephalus & Conjoined Twins⁵⁴:
    1. "Blatti (Stephan) - Animalism, Dicephalus, and Borderline Cases", 2007, Annotations, External Link, Internal PDF Link
    2. "Buchanan (Rachael) - The battle to separate Safa and Marwa", 2019, External Link, Internal PDF Link
    3. "Paris (J.J.) & Elias-Jones (A.C.) - Do we murder Mary to save Jodie?", 2001, Annotations, External Link
    4. "Walker (Robert) - Mary And Jodie – The Case Of The Conjoined Twins", 2002, Annotations, External Link
    5. "Wikipedia - Craniopagus Twins", 2017, Annotations, External Link, Internal PDF Link
  • General:
    1. "Blatti (Stephan) - Animalism and its Implications", 2005, Annotations, Internal PDF Link
    2. "Blatti (Stephan), Ed. - The Lives of Human Animals", 2014, External Link, Internal PDF Link
    3. "Hershenov (David) - Countering the Appeal of the Psychological Approach to Personal Identity", 2004, Annotations, External Link, Internal PDF Link
    4. "Liao (S. Matthew) - The Organism View Defended", 2006, Annotations, Internal PDF Link
    5. "Shoemaker (David) - Personal Identity and Immortality", 2009
    6. "Shoemaker (David) - Personal Identity, Rational Anticipation, and Self-Concern", 2009
    7. "Wilson (Jack) - Beyond Horses and Oak Trees: A New Theory of Individuation for Living Entities", 1999, Annotations
• A further reading list might start with:-
  • Dicephalus & Conjoined Twins⁵⁵:
    1. "Barilan (Y. Michael) - One or Two: An Examination of the Recent Case of the Conjoined Twins from Malta", 2003, Internal PDF Link
    2. "Campbell (Tim) & McMahan (Jeff) - Animalism and the Varieties of Conjoined Twinning", 2016, Read = 40%
    3. "Koch-Hershenov (Rose J.) - Conjoined Twins and the Biological Account of Personal Identity", 2006, Internal PDF Link, Read = 11%
    4. "Olson (Eric) - The Metaphysical Implications of Conjoined Twining", 2014, External Link, Internal PDF Link, Read = 17%
    5. "Reid (Mark D.) - A Case in Which Two Persons Exist in One Animal", 2016, Read = 42%
    6. "Stone (James L.) & Goodrich (James T.) - The craniopagus malformation: classification and implications for surgical separation", 2006, Annotations, External Link, Internal PDF Link, Read = 42%
    7. "Wikipedia - Abby and Brittany Hensel", 2023, External Link, Read = 25%
    8. "Wikipedia - Conjoined Twins", 2023, External Link, Read = 10%
    9. "Wikipedia - Craniopagus Parasiticus", 2023, External Link, Read = 25%
    10. "Wikipedia - Dicephalic Parapagus Twins", 2023, External Link, Read = 25%
    11. "Wikipedia - Diprosopus", 2023, External Link, Read = 25%
    12. "Wikipedia - Polycephaly", 2023, External Link, Read = 10%
  • General:
    1. "Hershenov (David) - Problems with a Constitution Account of Persons", 2009, External Link, Internal PDF Link, Read = 3%
    2. "Hershenov (David) - Shoemaker's Problem of Too Many Thinkers", 2007, External Link, Internal PDF Link
    3. "Snowdon (Paul) - Animalism and the Unity of Consciousness", 2014, Read = 7%
• For further papers held on-line of potential interest, follow this Link⁵⁶. Total papers = 5.

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• For a list of Works that have been considered, but have missed the cut for inclusion in this Section of my Thesis, see the following:-
  1. Read: No items to list.
  2. Further Reading: No items to list.

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In-Page Footnotes:

Footnote 7:

1. All this needs tidying up.
2. It’s probably best to take the Wikipedia article in full and link it to these reports and extract the relevant biological facts and cut out all the social stuff that is irrelevant to the metaphysical issues.

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Table of the Previous 12 Versions of this Note:

Date	Length	Title
06/07/2023 00:43:12	10751	Dicephalus
20/09/2022 11:24:00	9701	Dicephalus
11/05/2022 18:59:02	9360	Dicephalus
11/02/2021 11:29:46	8754	Dicephalus
13/05/2020 18:57:36	8102	Dicephalus
30/11/2019 22:58:05	8115	Dicephalus
30/07/2018 11:42:43	7784	Dicephalus
10/02/2018 14:15:22	6880	Dicephalus
04/07/2014 22:46:34	3633	Dicephalus
18/12/2010 19:58:05	394	Dicephalus
26/11/2007 23:25:26	398	Dicephalus
12/08/2007 10:17:46	389	Dicephalus

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Note last updated	Reading List for this Topic	Parent Topic
02/03/2026 06:44:06	Dicephalus	Clinical Observations

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Summary of Notes Referenced by This Note

Animalism	Animalism - Objections	Animals	Brain	Cerebrum
Counting Persons	Dicephalus	Duplication	Embryo	Fission
Fusion	Human Animals	Individual	Modality	Note Links Explanation
Organisms	Persistence	Person	Personality	PID Note, Book & Paper Usage
Psychology	Replication	Thought Experiments	Twinning	What are We?
Works Read - Explanation

To access information, click on one of the links in the table above.

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Summary of Notes Citing This Note

Animalism - Objections, 2, 3	Brain Transplants	Clinical Observations	Commissurotomy	Fission
Fusion	PID Note, Book & Paper Usage, 2, 3, 4	Status: Personal Identity (2025 - December)	Status: Priority Task List (2026 - February)	Status: Summary (2025 - December)
Thesis - Chapter 08 (Arguments against Animalism), 2, 3, 4, 5, 6, 7, 8, 9, 10	Thesis - Chapter 10 (Thought Experiments), 2, 3, 4, 5, 6	Thesis - Personal Identity, 2, 3, 4, 5, 6, 7, 8, 9	Twinning, 2	Website Generator Documentation - Functors, 2, 3, 4, 5, 6

To access information, click on one of the links in the table above.

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Authors, Books & Papers Citing this Note

Author	Title	Medium	Extra Links	Read?
Ayers (Michael R.)	Locke on Living Things	Paper		Yes
Barilan (Y. Michael)	One or Two: An Examination of the Recent Case of the Conjoined Twins from Malta	Paper	2
Blatti (Stephan)	Animalism and its Implications	Paper	2	Yes
Blatti (Stephan)	Animalism, Dicephalus, and Borderline Cases	Paper	2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14	Yes
Blatti (Stephan), Ed.	The Lives of Human Animals	Paper	2, 3	Yes
Buchanan (Rachael)	The battle to separate Safa and Marwa	Paper		Yes
Campbell (Tim) & McMahan (Jeff)	Animalism and the Varieties of Conjoined Twinning	Paper	2, 3, 4, 5
Hershenov (David)	Countering the Appeal of the Psychological Approach to Personal Identity	Paper	2, 3	Yes
Hershenov (David)	Problems with a Constitution Account of Persons	Paper	2
Hershenov (David)	Shoemaker's Problem of Too Many Thinkers	Paper
Kind (Amy)	How to think about consciousness	Paper		Yes
Kingma (Elselijn)	Were You Part of Your Mother?	Paper		Yes
Liao (S. Matthew)	The Organism View Defended	Paper	2, 3, 4, 5	Yes
Olson (Eric)	The Metaphysical Implications of Conjoined Twining	Paper
Reid (Mark D.)	A Case in Which Two Persons Exist in One Animal	Paper	2
Shoemaker (David)	Personal Identity and Immortality	Paper	2, 3, 4	Yes
Shoemaker (David)	Personal Identity, Rational Anticipation, and Self-Concern	Paper	2, 3, 4, 5, 6, 7, 8	Yes
Snowdon (Paul)	Animalism and the Unity of Consciousness	Paper	2, 3
Todman (Theo)	Thesis - Animalism - Objections	Paper	2, 3	Yes
Todman (Theo)	Thesis - Brain Transplants	Paper		Yes
Todman (Theo)	Thesis - Chapter 08 (Arguments against Animalism)	Paper	2, 3, 4, 5, 6, 7, 8, 9, 10	Yes
Todman (Theo)	Thesis - Chapter 10 (Thought Experiments)	Paper	2, 3, 4, 5, 6	Yes
Todman (Theo)	Thesis - Clinical Observations	Paper		Yes
Todman (Theo)	Thesis - Commissurotomy	Paper		Yes
Todman (Theo)	Thesis - Fission	Paper		Yes
Todman (Theo)	Thesis - Fusion	Paper		Yes
Todman (Theo)	Thesis - Personal Identity	Paper	2, 3, 4, 5, 6, 7, 8, 9	Yes
Todman (Theo)	Thesis - Twinning	Paper	2	Yes
Wilson (Jack)	Beyond Horses and Oak Trees: A New Theory of Individuation for Living Entities	Paper		Yes
Wyatt (John)	Matters of Life and Death	Book	2

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References & Reading List

Author	Title	Medium	Source	Read?
Barilan (Y. Michael)	One or Two: An Examination of the Recent Case of the Conjoined Twins from Malta	Paper - Cited	Journal of Medicine and Philosophy, Feb. 2003, 28.1, pp. 27-44	No
Blatti (Stephan)	Animalism and its Implications	Paper - Cited	OU Website (now deleted)	Yes
Blatti (Stephan)	Animalism, Dicephalus, and Borderline Cases	Paper - Cited	Philosophical Psychology 20 (2007): 595–608	Yes
Blatti (Stephan) & Snowdon (Paul), Eds.	Animalism: New Essays on Persons, Animals, and Identity	Book - Cited (via Paper Cited)	Bibliographical details to be supplied	22%
Blatti (Stephan), Ed.	The Lives of Human Animals	Paper - Cited	The Southern Journal of Philosophy Volume 52, Spindel Supplement, 2014	Yes
Buchanan (Rachael)	The battle to separate Safa and Marwa	Paper - Cited	BBC Website, July 2019	Yes
Campbell (Tim) & McMahan (Jeff)	Animalism and the Varieties of Conjoined Twinning	Paper - Cited	Blatti & Snowdon - Animalism: New Essays on Persons, Animals, and Identity, 2016: Part III, Chapter 11, pp. 229-252	40%
Hershenov (David)	Countering the Appeal of the Psychological Approach to Personal Identity	Paper - Cited	Philosophy - 79, Jul2004, Issue 309, p447-474, 28p;	Yes
Hershenov (David)	Problems with a Constitution Account of Persons	Paper - Cited	Dialogue. 48:2. 2009, 291-312	3%
Hershenov (David)	Shoemaker's Problem of Too Many Thinkers	Paper - Cited	Proceedings of the American Catholic Philosophical Association, 80, 2007, 225-36	No
Koch-Hershenov (Rose J.)	Conjoined Twins and the Biological Account of Personal Identity	Paper - Cited	The Monist, Vol. 89, No. 3, Coming into Being and Passing Away (July 2006), pp. 351-370	11%
Liao (S. Matthew)	The Organism View Defended	Paper - Cited	The Monist, Vol. 89, No. 3, Coming into Being and Passing Away (July 2006), pp. 334-350	Yes
O'Hear (Anthony), Ed.	Philosophy - 79/309 (July 2004)	Book - Cited (via Paper Cited)	Bibliographical details to be supplied	24%
Olson (Eric)	Papers on Identity Boxes: Vol 13 (Olson)	Book - Cited (via Paper Cited)	Bibliographical details to be supplied	16%
Olson (Eric)	The Metaphysical Implications of Conjoined Twining	Paper - Cited	Southern Journal of Philosophy 52, Spindel Supplement, 2014: 24-40	17%
Paris (J.J.) & Elias-Jones (A.C.)	Do we murder Mary to save Jodie?	Paper - Cited	Postgrad Med J 2001;77:593–598	Yes
Reid (Mark D.)	A Case in Which Two Persons Exist in One Animal	Paper - Cited	Blatti & Snowdon - Animalism: New Essays on Persons, Animals, and Identity, 2016: Part III, Chapter 12, pp. 253-265	42%
Shoemaker (David)	Personal Identity and Ethics: A Brief Introduction	Book - Cited (via Paper Cited)	Bibliographical details to be supplied	Yes
Shoemaker (David)	Personal Identity and Immortality	Paper - Cited	Shoemaker (David) - Personal Identity and Ethics: A Brief Introduction, 2009, Chapter 1	Yes
Shoemaker (David)	Personal Identity, Rational Anticipation, and Self-Concern	Paper - Cited	Shoemaker (David) - Personal Identity and Ethics: A Brief Introduction, 2009, Chapter 2	Yes
Snowdon (Paul)	Animalism and the Unity of Consciousness	Paper - Cited	Snowdon (Paul) - Persons, Animals, Ourselves - Chapter 8	7%
Snowdon (Paul)	Persons, Animals, Ourselves	Book - Cited (via Paper Cited)	Bibliographical details to be supplied	21%
Stone (James L.) & Goodrich (James T.)	The craniopagus malformation: classification and implications for surgical separation	Paper - Cited	Brain, Volume 129, Issue 5, May 2006, Pages 1084–1095	42%
Todman (Theo)	Thesis - Dicephalus	Paper - By Subtopic		Yes
Walker (Robert)	Mary And Jodie – The Case Of The Conjoined Twins	Paper - Cited	An address given by Sir Robert Walker, as he then was, to the Medico-Legal Society of Northern Ireland on 15 January 2002	Yes
Wikipedia	Abby and Brittany Hensel	Paper - Cited	Wikipedia; Extract taken 15 March 2023	No
Wikipedia	Conjoined Twins	Paper - Cited	Wikipedia; Extract taken 15 March 2023	10%
Wikipedia	Craniopagus Parasiticus	Paper - Cited	Wikipedia; Extract taken 15 March 2023	No
Wikipedia	Craniopagus Twins	Paper - Cited	Wikipedia; Extract taken 18 July 2017	Yes
Wikipedia	Dicephalic Parapagus Twins	Paper - Cited	Wikipedia; Extract taken 15 March 2023	No
Wikipedia	Diprosopus	Paper - Cited	Wikipedia; Extract taken 15 March 2023	No
Wikipedia	Polycephaly	Paper - Cited	Wikipedia; Extract taken 15 March 2023	10%
Wilson (Jack)	Beyond Horses and Oak Trees: A New Theory of Individuation for Living Entities	Paper - Cited	Wilson, Jack - Biological Individuality: The identity and Persistence of Living Entities; 1999, Chap. 1, pp. 1-21	Yes
Wilson (Jack)	Biological Individuality - The identity and Persistence of Living Entities	Book - Cited (via Paper Cited)	Bibliographical details to be supplied	Yes

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Text Colour Conventions

1. Blue: Text by me; © Theo Todman, 2026
2. Mauve: Text by correspondent(s) or other author(s); © the author(s)

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