There is a widespread practice of using evidence obtained from conceiving/imagining for establishing possibility claims. As a case study. I offer a critical reconstruction of an influential conceivability argument, Bohn’s (Philosophical Quarterly 59:193-201, 2009a, Analysis 70:296–298, 2009b, Analysis 70:296-298, 2010) argument for junk (there are proper parts forming wholes, but there is no whole that is not a proper part itself). The most influential scenario used by Bohn and his successors is purported to be junk but not ensured to be (...) junk. This finding calls for an explanation. I distinguish ways of conceiving/imagining that are insufficient to establish modal claims from positive conceiving/imagining, which is more promising. Positive conceiving/imagining often works by a partial structural isomorphy to its target, and it is sufficiently detailed. In order to be sufficiently detailed, it has to properly integrate theoretical considerations. I outline a concern about extant ways of conceiving/imagining junk: they do not yet include a metaphysical understanding of what it takes for there to be proper parts and wholes without a whole that is not a proper part itself, for instance in terms of a non-gerrymandered principle of composition. (shrink)