To conceive of a world W in which proposition (i) is true is to conceive of a world in which we (in the actual world) attribute beliefs about mind-independent objects to counterfactual versions of us (CVs) in W.
attribute (as a verb)(as used in logic and language analysis)
To say that something has a particular quality or characteristic—like saying a painting is 'colorful' attributes the quality of being colorful to that painting.
mind-independent objects(What Berkeley's Master Argument aims to show we cannot coherently conceive)
Objects that exist without being perceived or conceived by any mind; objects whose existence does not depend on being represented in consciousness
proposition(Used in the context of a semantic theory sensitive to differences in subject matter.)
The content expressed by a sentence, individuated at least in part by the subject matter of the sentence and the contents of its subsentential expressions.
Stroud argues, in effect, that given CT we cannot believe that (i) and (ii) are true, but nonetheless, we can believe that (i) and (ii) are logically compossible given CT. Brueckner argues that using similar reasoning, given CT we cannot conceive of a possible world in which both (i) and (ii) hold, and this fact undermines Stroud’s claim. To conceive of a world W in which (i) is true is to conceive of a world in which we (in the actual world) attribute beliefs about mind-independent objects to c