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Notes
Philosophical Studies, 18:45–47 (1967).
Ibid., pp. 45–46.
In general the argument form\(\frac{\begin{gathered} 'P \supset \square Q' \hfill \\ P \hfill \\ \end{gathered} }{{\square Q}}\) is invalid because it purports to derive a necessary conclusion from two contingent premises, but also the first premise is unacceptable because it claims that a necessary proposition follows from a contingent one.
Plato, in thePhaedo, 105A–107B, recognized that to show that it is necessary that the soul is alive is insufficient to prove that it is immortal in the usual sense. He did claim, however, that ‘x cannot admit death’ (the unusual sense of ‘immortal’) entails ‘x can never perish’ (the usual sense of ‘immortal’). The argument depends, I think, upon the equivocal use of ‘immortal.’ His argument is as follows: (1) Forms do not admit opposite Forms. (2) Anything accompanied by a Form which has an opposite cannot admit the opposite; it either ceases to be or it withdraws upon the approach of the opposite Form. (3) The soul is accompanied by the Form life. (4) The opposite of life is death. (5) The soul cannot admit death. (6) Therefore, the soul is immortal. (7) Whatever is immortal is also imperishable. (8) Therefore, the soul is immortal and imperishable. In steps 2, 7, and 8 Plato is contrasting the soul with such things as snow, which is accompanied by a Form, cold, and cannot admit the opposite Form, heat, but which ceases to be rather than withdraws on the approach of the opposite. In other words, Plato saw clearly the distinction between ‘it is necessary that x has P’ (when P is an essential attribute of x) and ‘x cannot perish.’ From the claim ‘something cannot both be an x and not-P,’ we cannot infer ‘x cannot perish.’ That is, from the claim that something cannot be both a person and not alive, we cannot infer that persons are immortal. Plato, however, uses ‘immortal’ in an equivocal sense. In (6) it means simply ‘cannot die,’ whereas in (7), it means what is usually meant, i.e., ‘cannot perish.’ And as he has pointed out, something which cannot admit death may perish.
‘Philosophical Studies, 18:46.
Ibid.
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Klein, P. “Are Strawson’s Persons immortal?” A reply. Philos Stud 20, 65–70 (1969). https://doi.org/10.1007/BF02316966
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DOI: https://doi.org/10.1007/BF02316966