Discourse on Method
The Immateriality of the Soul
(1) …si j’eusse seulement cessé de penser, encore que tout le reste de ce que j’avais imaginé eût été vrai, je n’avais aucune raison de croire que j’eusse été….
Which is immediately followed by:
(2) …je connus de là que j’étais une substance dont toute l’essence ou la nature n’est que de penser, et qui, pour être, n’a besoin d’aucun lieu, ni dépend d’aucune chose matérielle. (p. 148)
What appears to differentiate this argument from anything in the Meditations is that it contains a unique reason for the conclusion that the soul is immaterial. Descartes (His meaning is ambiguous) asserts either that when we are not thinking we cannot prove that we exist, or that we cannot prove that we exist when (i.e. at those times when) we are not thinking. Whichever of these two premises he really intends by (1), he concludes from it that we are our thinking and so immaterial.
Now (1) can mean any one of several different things (It is confusing because of mixed moods: Two indicatives rub shoulders with three subjunctives). It can mean:
(1a) If I cease to think, then I have no reason to believe that I exist.
Or it can mean:
(1b) When I cease to think, then I have no reason to believe that I exist.
(1c) I have no reason to believe that I exist on those occasions when I do not think.
(1d) I have no reason to believe that I exist if I do not think.
None of these possible meanings of (1) follow from the strict argument of the Cogito unless Descartes means that I have no reason to believe that I continue to exist if I cease to think. For, once I follow and accept the argument of the Cogito, then I have, by that argument, reason to believe that I exist (or existed) whether or not I cease to think after I have followed and accepted the argument of the Cogito. I may not know that I have reason to believe that I exist(ed), because I am not thinking, but that does not mean that I don’t have reason to believe that I exist(ed). (In the final accounting, (1) introduces a temporal element that, because the Cogito is enunciated without temporal reference, actually invalidates the Cogito for any time that I am not actually concentrating on and accepting the argument. Even if the Cogito were to prove that I exist now, it does not prove that I existed yesterday.)
But, even assuming that (1) follows from the Cogito, (2) does not follow from (1), for (2) is a much stronger claim than (1). (1) simply states that I am a thing that has no reason to believe that it exists except when it thinks. (2) states that I am essentially a thinking thing and goes on to say that I would continue to exist even if every aspect of my materiality were to be taken away. The actual conclusion of the Cogito is much weaker than that. It states that I have no certain knowledge that I am material even when I think. (2), however, states that I have certain knowledge that I am not material because I can doubt that I am material. To get from the Cogito to (2) you have to accept the obviously unacceptable view that for all propositions, p, if I do not have certain knowledge that p, then p is false. In other words, I may be thinking matter, but I just don’t have indubitable proof that I am thinking matter. In other words,
(3) I can’t be sure that I am material even when I think,
does not entail:
(4) I am not material even when I think.
Finally, the version of the argument for the non-materiality of the soul in the Discourse goes from
(5) I can’t be sure that I exist if (when) I don’t think
(6) I don’t exist if (when) I don’t think.
But clearly (5) does not entail (6).