 A rather different line of approach to the epistemological problem was proposed in Linsky &amp; Zalta 1995, where it is suggested that one shouldn’t attempt to explain knowledge of abstracta on the same model that is used to explain knowledge of concrete objects. They argue that not only a certain plenitude principle for abstract objects (namely, the comprehension principle for abstract objects put forward in Zalta 1983, 1988—see below) yields unproblematic ‘acquaintance by description’ to unique abstract objects but also that their approach actually comports with naturalist beliefs. Balaguer (1995, 1998) also suggests that a plenitude principle is the best way forward for the platonist, and that our knowledge of the consistency of mathematical theories suffices for knowledge of mathematical objects. And there are views that conceive of abstract objects as constituted by human—or, in general, intelligent—subjects, or as abstract artifacts (see Popper 1968; Thomasson 1999).