  The argument seems intended to establish that we cannot actually conceive of mind-independent objects, that is, objects existing unperceived and unthought of. Why not?  Simply because in order to conceive of any such things, we must ourselves be conceiving, i.e., thinking, of them. However, as Pitcher (1977, 113) nicely observes, such an argument seems to conflate the representation (what we conceive with) and the represented (what we conceive of—the content of our thought). Once we make this distinction, we realize that although we must have some conception or representation in order to conceive of something, and that representation is in some sense thought of, it does not follow (contra Berkeley) that what we conceive of must be a thought-of object. That is, when we imagine a tree standing alone in a forest, we (arguably) conceive of an unthought-of object, though of course we must employ a thought in order to accomplish this  feat.[6]   Thus (as many commentators have observed), this argument fails.