I’ve been discussing Descartes’ version of the replacement argument with a chap here, and since I can’t find an online copy of the dialogue I’m going to type out the relevant passage from a printout. This is from a book no one will probably ever see again, but I printed off the passage from a friend who stumbled upon it.
Elizabeth: How are they connected?
Descartes: Well, it’s really to do with possibilities. If it is possible for thinking to go on apart form a body then…
E: [Interrupting] But is it possible? That’s the question.
D: All right, I’m coming to that. I did say “if”. If it is possible for thinking, and the body, to exist in separation then…
E: [Impatiently] Yes, yes, then what-does-the-thinking isn’t the body. I can quite see that. But what you’ve got to do is to get rid of the “if”. That is, you’ve got to show it to be possible for thinking to go on apart from a body.
D: Precisely, and that is where what I know and don’t know, comes in.
E: Go on.
D: Well, I know certainly that I am thinking and at the same time I can doubt that I have bodily attributes. So I can perceive the one thing, the thinking, apart from the other. And since this perception is clear and distinct it must be possible for the one thing to exist apart from the other.
E: Just a moment. You said :since this perception is clear and distinct.”
E: And therefore you really are no more than a thinking thing?
E: All right. Well now, isn’t is possible that your perception is clear, but only as far as it goes? And that it doesn’t go far enough for you to know the truth? In other words, isn’t it possible that you really do have bodily properties although your knowledge of yourself doesn’t go beyond your mental properties.
D: No. You must distinguish between clearness and completeness. Certainly there may be things about me which I haven’t clearly perceived. But that doesn’t affect what I have clearly perceived. And, having clearly perceived that I am a thinking thing, I know that I can exist as such. That is, I know that what I am certain of- my intellectual faculty- is enough for me to exist with. And if it is enough for me to exist with, then I really am distinct from anything bodily.
E: So, the principle of your argument is: if I can clearly perceive something to be such-and-such while I cannot clearly perceive it to be so-and-so, then it can exist simply as such-and-such.
This is a fascinating exchange that, as far as I can tell, has many components of the discussion that has cropped up around Plantinga’s version. More on that as we progress into an analysis of Plantinga’s presentation of the argument.