 Some ideas, called objectual (gegenständlich) by Bolzano, have, or represent, one or more objects, for instance, [the Bolzano-Weierstrass theorem], [raven]. Others are objectless (gegenstandlos), e.g., [round square], [golden mountain], [not] (TS, §67). Not recognizing empty collections, Bolzano says that objectless ideas have no extensions.