A Refutation of Metaphysical Idealism?

K. G. presents me with what he calls a conceivability argument against metaphysical idealism:

Let P denote the proposition "I have a body." Then the argument would take the form

1. P is conceivable. 

2. If P is conceivable, then P is possible.

3. If P is possible, then metaphysical idealism is false.

Therefore, metaphysical idealism is false. 

 

Premise 1 is uncontroversial because I can see what I consider to be my body, and thus I can form a mental image of it. Premise 3 merely follows from the definition of idealism. Premise 2 is the most controversial, but I think that replacing "conceivable" with "imaginable" will avoid all difficulties associated with this premise. I may be able to conceive of a triangle which is neither isoceles nor scalene, but I cannot imagine one. 

 

What do you think?

 

I have two objections.  

 

1. You appreciate that there is a problem with validating the inferential move from 'x is conceivable' to 'x is possible.'  But you think the move from 'x is imaginable' to 'x is possible' is unproblematic.  I disagree.  Suppose we agree that 'x is imaginable' means 'There is a human person who has the ability to form a mental image of x.'  If this is what we mean by 'imaginable,' then all sorts of things are imaginable that are not possible.  For example, I have just now formed the mental image of an ordinary tire iron floating in ordinary water.   But this is not a nomologically possible state of affairs: it is ruled out by the (logically contingent) laws of nature. 

 

You need to be careful not to confuse the image with what the image is of or about. The image of floating iron is of course an actual image and therefore a possible image.  The question, however, is whether what the image depicts is possible.   The mere fact that one can form a mental image of x does not show that x is possible.  For the image of x is not x.  

 

To this you might respond that you have in mind broadly logical possibility, not nomological possibility.  Take a gander at this M.C. Escher drawing:

 

What you see is an object of visual perception, but you could imagine the hands easily enough, as presumably Escher himself did before he made the drawing.  What the image is of, however, is broadly logically impossible.  Two right hands are depicted each of which comes into existence by being drawn by the other.

 

But apart from examples, why should possibility be tied to what we can conceive or imagine?  Our powers of conception and imagination are limited.  Besides, if I have the power to imagine such-and-such, then it must be possible that I imagine such-and-such in which case it would be circular to explain possibility in terms of imaginability.

 

2. Philosophers are not in the business of denying obvious facts.   It is an obvious fact that I have a body. It follows straightaway that it is possible that I have a body.  But this possibility does not refute idealism.   For the obvious fact that there are bodies can be interpreted both realistically and idealistically.  A metaphysical idealist such as Berkeley does not deny that there are bodies; he proffers a theory as to what bodies are in their ontological structure.  At ontological bottom there are only minds and ideas in the Berkeleyan system, with physical things construed as collections of ideas.  His line on bodies is not nihilist or eliminativist, but reductivist: bodies reduce to collections of ideas.  For this reason I would reject your premise (3).

 

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