Gottlob Frege’s On Sense and Reference (Über Sinn und Bedeutung, 1892) is concerned with the question of how the sense (or mode of presentation) of a sign is related to the meaning which is expressed by the sign. Frege examines the question of how the object which is designated by a sign is related to the meaning which is expressed by the sign. He explains that changes in the sense (or mode of presentation) of a sign may change the meaning of the sign, and he describes how changes in the sense of a sign may also change the sense of the expression in which the sign is contained. Thus, he discusses the relation between sense and reference, and he examines the relation between signification and meaning.
Frege defines a sign as any name, word, combination of words, or expression which may be used to designate an object. The sense of a sign is the part of its meaning which depends on its mode of presentation. The meaning of a sign is to be distinguished from the object to which the sign refers. The object to which a sign refers may provide the basis for the sign's dimension of reference. However, the meaning of a sign may have both a dimension of sense and a dimension of reference.
Frege explains that the mode of presentation of a sign and the mode of reference of the sign to an object may correspond to each other, but the sense of a sign may change without changing the identity of the object to which the sign refers. The identity of the object to which a sign refers may be independent of the way in which the sign is presented.
Frege also explains that an object may be designated by many different signs, and that different signs which designate the same object may have different meanings. Every sign which refers to an object may not necessarily have a sense or meaning. Every sign which has a sense or meaning may not necessarily refer to a definite object. The object which is designated by a sign may not necessarily be capable of being defined.
As an example of how different signs for the same object may have different senses, Frege notes that the term 'morning star' and the term 'evening star' both refer to the same object, although these terms have different senses. The term 'morning star' refers to the planet Venus as it appears at dawn, while the term 'evening star' refers to the planet Venus as it appears at dusk.
Frege argues that the designation of any object by a particular sign may be arbitrary, and that any name or expression may be arbitrarily chosen to designate a particular object. Signs are themselves objects, and thus may themselves be designated by other signs. The connection between a sign and the object which it designates may not necessarily be explained by any quality of the object.
According to Frege, the discovery that different signs may be used to designate the same object may in some cases enable us to attain additional knowledge of the object which is designated by those signs. To discover all of the possible signs which may be used to designate an object may be to attain all possible knowledge of that object, but such comprehensive knowledge of an object may not be within our capacity to attain.1
Frege explains that the object to which a sentence refers is either that which is true or that which is false. The object to which a sentence refers is thus its truth-value, i.e. the set of conditions which determine its truth or falsehood. Every declarative sentence is either a sign for that which is true or a sign for that which is false. Declarative sentences are signs for truth-values and may designate these truth-values in many different ways.
Frege also explains that the thought which is expressed by any declarative sentence has the same relation to truth and falsehood as sense has to reference. The truth-value of a sentence is not determined by the mode of presentation of the sentence (or by the sentence's dimension of sense), but is defined by the snetence's dimension of reference. Thus, in order to determine whether the thought which is expressed by a sentence provides us with any valid knowledge, we must be able to determine not only the meaning of the sentence but its truth-value.
According to Frege, an expression within a sentence may be changed without changing the truth-value of the sentence if the expression is replaced by another expression which has the same truth-value or which designates the same object. However, an exception to this general rule may occur when a subordinate clause of a complex sentence does not have a truth-value and is replaced by another subordinate clause which does not have a truth-value. The truth-value of the whole sentence may be changed even though no change has occurred in the truth-value of each subordinate clause.
Frege explains that in some cases a subordinate clause in a complex sentence may refer to a thought which is expressed by the main clause of the sentence. The subordinate clause may thus not have a truth-value of its own, and may refer only indirectly to the set of conditions for truth and falsehood which determine the truth-value of the whole sentence. The subordinate clause may not express a complete thought, and may therefore not have any definite meaning apart from the way in which it contributes to the meaning of the whole sentence.
Frege also explains that in a logically perfect language, every proper name or sign designates an object.2 In a logically imperfect language, every proper name or sign does not designate an object. Logical errors in langauage may be caused by using ambiguous signs or by using signs which do not refer to definite objects. Logical errors may be avoided by not using signs which do not refer to definite objects.
1Gottlob Frege, "On Sense and Reference," in Translations from the Philosophical Writings of Gottlob Frege, edited by Peter Geach and Max Black (Oxford: Basil Blackwell, 1960), p. 58.
2Ibid., p. 70.
Collinson, Diané. "Gottlob Frege," in Fifty Major Philosophers: A Reference Guide. London: Routledge (1987), pp. 125-8.
Dummett, Michael. "Gottlob Frege," in The Encyclopedia of Philosophy. Edited by Paul Edwards. New York: Crowell, Collier and MacMillan (1967), pp. 225-37.
Frege, Gottlob. "On Sense and Reference," in Translations from the Philosophical Writings of Gottlob Frege. Edited by Peter Geach and Max Black. Oxford: Basil Blackwell, 1960.
Frege, Gottlob. "Über Sinn und Bedeutung,” in Zeitschrift für Philosophie und philosophische Kritik, Vol. 100 (1892), pp. 25-50.