- This pseudo-Paper is intended as the mechanism to record time spent on the Note 'Dicephalus1' during my Thesis research, as from 2011.
- For the actual time recorded, click on "Paper Statistics" above.
Write-up2 (as at 14/04/2026 03:53:17): Dicephalus
Notes
- Thesis Text:
- 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 animals3 and the number of individuals4 may fail to align. In order of increasing severity of the “condition”:-
- 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 fission5 versus fusion6, with the latter now the favourite suggestion.
- Craniopagus: "Wikipedia - Craniopagus Twins". The babies are joined at the skull. Contrast the case where both twins are persons7 with the degenerate case "Wikipedia - Craniopagus Parasiticus".
- 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 fission8 of an original single individual – this descriptive decision may be correct.
- Diprosopus: "Wikipedia - Diprosopus".
- 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.
- For more on diprosopus9 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.
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 duplicated10 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 replicated11, 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.
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.
All these situations – and in particular the paradigm case of dicephalus, wherein it appears that we have a human animal12 with two heads – are often cited as a prima facie problems for Animalism13.
The claim is that while we have multiple persons14, we only have a single animal15, so the animalist16 claim that persons are numerically identical to their organisms17 is false.
Of course, the above claim is somewhat bald. Animalism18 – at least in the form expounded by Eric Olson – has no truck with psychology19 as necessary for our persistence20. While Olson often refers to “people” this is just shorthand for “some of us21”, rather than “persons”, the plural of person22.
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 us23 are present, and how many human animals24 are present. Animalism25 needs the numbers to be equal.
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.
The cases under consideration have the advantage over standard Thought Experiments26, 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 possible27 because the TEs are under-described.
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 TEs28.
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 persons29 or two of “us30”. However, the case of the dicephalus twins31, 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 persons32, but maybe only one animal33. This is probably a real-life case of either incomplete fission34 or of fusion35.
There are two issues with these cases:-
- How should they be best described?
- How should the animalist respond?
Description:
- 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.
- I presume, also, that we can have either fission36 or fusion37, or indeed both serially – as the case may be – an empirical matter.
- In the case of (incomplete) fission38, the twins39 will be identical, as the situation is one where the twinning process has not completed.
- But (I imagine) we can also have a case of fusion40, where the twin-embryos41 – 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 fission42 followed by fusion43. I don’t know whether there have been any actual cases of this.
Response:
- The most likely response from the animalist44 is that the dicephalus is not a single organism45, but two organisms that share some body-parts.
- 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.
- In the case of diprosopus, it is more difficult to claim that there are two organisms46. Now – in practice – there are never two persons47 either (as in all the 30-odd cases recorded since 1860, the condition has proved quickly fatal, so there were no persons48 present). But this might not always be so. Also, Lynne Rudder Baker might claim that the rudimentary persons were indeed persons49 – 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 personality50. 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 ‘brains51’ (presumably two sets of cerebral hemispheres52). There don’t seem to have been any actual cases of this in humans, but there might have been53.
Further Remarks:
- 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:
- "Wikipedia - Abby and Brittany Hensel", 2023, External Link, Read = 25%
- "Wikipedia - Conjoined Twins", 2023, External Link, Read = 10%
- "Wikipedia - Craniopagus Parasiticus", 2023, External Link, Read = 25%
- "Wikipedia - Craniopagus Twins", 2017, Annotations, External Link, Internal PDF Link, Read
- "Wikipedia - Dicephalic Parapagus Twins", 2023, External Link, Read = 25%
- "Wikipedia - Diprosopus", 2023, External Link, Read = 25%
- "Wikipedia - Polycephaly", 2023, External Link, Read = 10%
- For a page of Links54 to this Note, Click here.
- Works on this topic that I’ve actually read55, include the following:-
- Dicephalus & Conjoined Twins56:
- "Blatti (Stephan) - Animalism, Dicephalus, and Borderline Cases", 2007, Annotations, External Link, Internal PDF Link
- "Buchanan (Rachael) - The battle to separate Safa and Marwa", 2019, External Link, Internal PDF Link
- "Paris (J.J.) & Elias-Jones (A.C.) - Do we murder Mary to save Jodie?", 2001, Annotations, External Link
- "Walker (Robert) - Mary And Jodie – The Case Of The Conjoined Twins", 2002, Annotations, External Link
- "Wikipedia - Craniopagus Twins", 2017, Annotations, External Link, Internal PDF Link
- General:
- "Blatti (Stephan) - Animalism and its Implications", 2005, Annotations, Internal PDF Link
- "Blatti (Stephan), Ed. - The Lives of Human Animals", 2014, External Link, Internal PDF Link
- "Hershenov (David) - Countering the Appeal of the Psychological Approach to Personal Identity", 2004, Annotations, External Link, Internal PDF Link
- "Liao (S. Matthew) - The Organism View Defended", 2006, Annotations, Internal PDF Link
- "Shoemaker (David) - Personal Identity and Immortality", 2009
- "Shoemaker (David) - Personal Identity, Rational Anticipation, and Self-Concern", 2009
- "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 Twins57:
- "Barilan (Y. Michael) - One or Two: An Examination of the Recent Case of the Conjoined Twins from Malta", 2003, Internal PDF Link
- "Campbell (Tim) & McMahan (Jeff) - Animalism and the Varieties of Conjoined Twinning", 2016, Read = 40%
- "Koch-Hershenov (Rose J.) - Conjoined Twins and the Biological Account of Personal Identity", 2006, Internal PDF Link, Read = 11%
- "Olson (Eric) - The Metaphysical Implications of Conjoined Twining", 2014, External Link, Internal PDF Link, Read = 17%
- "Reid (Mark D.) - A Case in Which Two Persons Exist in One Animal", 2016, Read = 42%
- "Stone (James L.) & Goodrich (James T.) - The craniopagus malformation: classification and implications for surgical separation", 2006, Annotations, External Link, Internal PDF Link, Read = 42%
- "Wikipedia - Abby and Brittany Hensel", 2023, External Link, Read = 25%
- "Wikipedia - Conjoined Twins", 2023, External Link, Read = 10%
- "Wikipedia - Craniopagus Parasiticus", 2023, External Link, Read = 25%
- "Wikipedia - Dicephalic Parapagus Twins", 2023, External Link, Read = 25%
- "Wikipedia - Diprosopus", 2023, External Link, Read = 25%
- "Wikipedia - Polycephaly", 2023, External Link, Read = 10%
- General:
- "Hershenov (David) - Problems with a Constitution Account of Persons", 2009, External Link, Internal PDF Link, Read = 3%
- "Hershenov (David) - Shoemaker's Problem of Too Many Thinkers", 2007, External Link, Internal PDF Link
- "Snowdon (Paul) - Animalism and the Unity of Consciousness", 2014, Read = 7%
- For further papers held on-line of potential interest, follow this Link58. Total papers = 5.
- 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:-
- Read: No items to list.
- Further Reading: No items to list.
In-Page Footnotes
Footnote 2:
- This is the write-up as it was when this Abstract was last output, with text as at the timestamp indicated (14/04/2026 03:53:17).
- Link to Latest Write-Up Note.
Footnote 9:
- All this needs tidying up.
- 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.
Text Colour Conventions (see disclaimer)
- Blue: Text by me; © Theo Todman, 2026
- Mauve: Text by correspondent(s) or other author(s); © the author(s)