What presents itself as meaningful is not always so; and what at first seems meaningless can be deeply significant. “Lead Rhetoric & Other Category Errors,” the meaning-laden title of this exhibition by Joanne Tatham and Tom O’Sullivan, who live in Glasgow and have been collaborating since 1995, already hinted at this contradiction. But what rhetoric was at work here, and what errors were in question? The rhetoric was that of Minimalism, and sometimes of Land art. Using the same form in various contexts, the artists might place a large cube in a landscape, or a smaller one in a museum on a