Evaluation of Example 13: Immortal Soul


“[1] The soul is ... no more capable of ... being divided in itself, because [2] it is not dissolvable. For [3] if it were compounded [3] it would be capable of dissolution, and [5] if it were capable of dissolution [6] it would not be immortal. Therefore, because [7] it is not mortal [8] it is neither dissolvable nor divisible.”

Charles S. Stanford, “Opinions on the Immortality of the Soul" in his translation of Phædo by Plato (New York: William Gowans: 1854),148.


The argument is not a petitio principii even though part of the final conclusion appears in a subargument. The argument can be presented as follows:

1st Modus Ponens:
[5] If the soul were dissolvable, it would not be immortal.
[7] The soul is not mortal.
[2] The soul is not dissolvable.
2nd Modus Tollens:
[3] If the soul is compounded, it is dissolvable.
[2]The soul is not dissolvable.(from step 1)
[The soul is not compounded.]
3rd Definition and AEE-1 Syllogism:
[All compounded things are divisible.] (from definition)
The soul is not compounded. (from step 2)
[1] The soul is not divisible.
4th Conjunction:
Therefore, the soul is not dissolvable and the soul is not divisible. (from steps 1 and 2)
Rule of Inference:
The soul is neither dissolvable nor divisible. (from step 4)