In his famous paper ‘Über Sinn und Bedeutung’ (‘On Sense and Reference’, 1892), Frege contrasted the sense (Sinn) of an expression with its reference (Bedeutung). Two expressions might have the same reference, but present it in different ways, and this mode of presentation is the sense of the expression. Thus ‘George Eliot’ and ‘Mary Anne Evans’ refer to the same person, but it might come as a surprise to someone to learn that the person he knows as one is the person he knows as the other. This, according to Frege, is because the terms are associated with different modes of presentation of the one person. It is the sense of expressions that determine the thought expressed by a sentence in which they occur, whilst reference determines its truth or falsity. However, there are sentences in which the mode of presentation or sense itself becomes part of the topic, in which case truth-value can shift through substitution of terms that nevertheless have the same reference. Such contexts are called oblique, or indirect, or intensional (see extension/intension). Two conditions on the notion of sense are that if two expressions share the same sense it should not be possible to fail to realize that they share the same reference; also senses are to be public and objective, as thoughts are for Frege. It has proven hard to identify a conception of sense that satisfies both these conditions. The use of a two-part semantic theory of this kind was attacked by Russell in his paper ‘On Denoting’ (1905), but theories of meaning are commonly based upon some version of the distinction. See also extension/intension.