- The rare condition known as ‘dicephalus1’ occurs when (prior to implantation) a zygote2 fails to divide completely, resulting in twins who are conjoined below the neck. Human dicephalic twins look like a two-headed person, with each brain supporting a distinct mental life.
- Jeff McMahan3 has recently argued that, because they instance two of us but only one animal, dicephalic twins provide a counter-example to the animalist’s claim that each of us is identical with a human animal.
- To the contrary, I argue that in cases of dicephalus it is obvious neither that there is one animal nor that there are two of us. Consequently, the animalist criterion does not straightforwardly apply to cases of dicephalus.
- I conclude by proposing a characterization of dicephalus that is both sensitive to the complexity of twinning phenomena and not inconsistent with animalism. On my view, dicephalic twins are a borderline case of the concept ‘human animal’.
Sections / Structure
- The Burden of Duplication Objections
- 2.1 The Double-Hemispherectomy Transplant Objection Meets its Burden
- Assumption objected to: Psychological Continuity Criterion
- Rejoinders rejected:-
- Non-branching condition on Psychological Continuity Criterion
- Lewis4 / Roland Puccetti (always two coincident persons)
- Nozick5 (closest continuer6)
- Johnston7 (indeterminate)
- Wilkes8 (reject thought experiments)
- 2.2 The Dicephalus Objection Does Not Meet its Burden
- (1) In cases of dicephalus, there exists (exactly) one animal.
- (2) In cases of dicephalus, there exist (exactly) two of us.
- Are (1) and (2) Obviously True?
- 3.1 Alternatives to (2)
- (2′) In cases of dicephalus, there exist (exactly) two subjects of experience.
- (2′′) In cases of dicephalus, there exist (exactly) two minds.
- (2′′′) In cases of dicephalus, there exists (exactly) one divided mind.
- 3.2 The Correspondence Intuition
- The presence of human twins is sufficient for the presence of two of us.
- 3.3 The Fusion Proposal
- Dicephalus as a Borderline Case
- Actually a diversion to reject forensic arguments
Footnote 3: "McMahan (Jeff) - The Ethics of Killing: Problems at the Margins of Life", 2002.
Footnote 4: "Lewis (David) - Survival and Identity", 1976.
Footnote 5: "Nozick (Robert) - Philosophical Explanations", 1981.
Footnote 7: "Johnston (Mark) - Fission and the Facts", 1989.
Footnote 8: "Wilkes (Kathleen) - Real People: Personal Identity Without Thought Experiments", 1988.
Text Colour Conventions (see disclaimer)
- Blue: Text by me; © Theo Todman, 2018
- Mauve: Text by correspondent(s) or other author(s); © the author(s)