'Phenomenalism' is the name given to Berkeley's doctrine that the being (existence) of material objects consists in their being perceived (their esse is percipi - PHK 3). This formula is, however, several ways ambiguous. Here I just want to point out one of them. (I have been thinking about these issues in connection with a paper I am writing on the question of whether Leibniz was a phenomenalist, and, if so, of what sort.) The ambiguity I am concerned with here is a de re/de dicto ambiguity.
De re is Latin for 'concerning the thing', and de dicto is Latin for 'concerning the saying.' De re/de dicto ambiguities are common in natural language. For instance, consider the sentence:
Necessarily, the President of the United States is commander-in-chief of the armed forces.
On the other hand, our example sentence is clearly false de re: the man who is in fact President of the United States, Barack Obama, is only contingently commander-in-chief. In fact, just a few years ago, he wasn't.
Now, there is, as I have said, a similar ambiguity in Berkeley's formula. On the de re interpretation, phenomenalism affirms, of every object which is in fact a body (e.g. my desk) that it's existence depends on perception. On the de dicto reading, phenomenalism simply affirms that if nothing were perceived, there wouldn't be any bodies.
These two doctrines are logically independent. Someone who said that all of the bodies that in fact exist are such that they could not exist without being perceived, but that there could have been other bodies which didn't depend on perception would be affirming de re phenomenalism but denying de dicto phenomenalism. Someone who said that the objects which are in fact bodies would still exist, but wouldn't be bodies, if they weren't perceived (as on some 'one world' interpretations of Kant) would be affirming de dicto phenomenalism, but denying de re phenomenalism. Berkeley (and, I am arguing in my paper, also Leibniz) affirms both, and he does this by holding that de re phenomenalism is a necessary truth. The proposition:
Necessarily, if anything is a body, then the existence of that thing depends on perception
If nothing were perceived, then nothing would be a body
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