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Sentences, Statements and Propositions
Various philosophical considerations have led philosophers to draw a distinction between these three things: sentences, statements and propositions. One consideration has been the question of what kind of thing has the property true or false, and another consideration has been the question of what the objects of belief and other propositional attitudes are. Because it is a complicated area, the three terms are not always used in the same way everywhere. However, here is a broad delineation:
A sentence is usually used to mean a grammatically correct and complete string of expression of a natural language. There are sentences types and sentence tokens. A sentence token is, for instance, the physical event of the utterance being made, or the physical marks inscribed on a page. A sentence type is harder to define, since some people mean the similarity of form, say phonological or orthographical, while others mean the sameness of meaning, so that for instance sentences in different languages which mean the same thing are of the same type. A statement usually means that which is said when a sentence is uttered or inscribed. Not all sentences make statements, for instance imperative or interrogative sentences, or sentences uttered in, say, reciting a play. A proposition is the hardest to define, but can be taken to mean that which is common to a set of synonymous declarative sentences. Propositions, even the false ones, are usually taken to exist timelessly and independently of anything that expresses them, and even independently of whether they are ever expressed.
For simplicity, I am going to investigate the way three particular philosophers have each dealt with one of the three notions, namely the one that they have chosen as the truth-bearer. These will be: Frege's propositions, Strawson's statements and Quine's sentences.
Frege's term Gedanke means 'thought', and it is to thoughts that Frege believes truth or falsity can apply. A thought is the sense of a sentence, although not every sense of a sentence need be a thought. "When we call a sentence true we really mean its sense is. From which it follows that it is for the sense of a sentence that the question of truth arises in general...I call a thought something for which the question of truth arises." Frege's thoughts correspond to what we call propositions -they are motivated by a lot of the same argument that someone arguing for propositions would give, and have the same important features.
Indicative sentences, as opposed to imperative sentences or 'Wh-' question sentences, can express thoughts. However, whole-sentence questions such as 'Is snow white?' can express thoughts, because one can grasp a thought without necessarily laying it down as true. The tone and feeling of an expression do not affect what thought is expressed, because they do not affect its sense, and they do not touch what is true or false. That these kinds of superficial transformations can be ignored is what allows for 'deeper logical investigation'; presumably Frege means the study of propositional calculuses and other such formal logics. The time and context and accompanying body language of an utterance can affect what thought is expressed, because these things make it clear what the sentence is about. Only a sentence supplemented by a time-indication and complete in every respect expresses a thought.
Now, thoughts present more of an ontological problem than their more mundane counterparts, sentences, for they are not linguistic items but a part of reality. In trying to discern the ontological status of thoughts, Frege comes to a sort of Platonic conclusion. He reasons that they are not material things, or objects of the senses, so he wonders whether they are kinds of ideas, which 'belong to the inner world'. Ideas, however, need a bearer. And Frege worries that if thoughts are ideas - things that take place in the mind and only occur in one's own sphere of consciousness - then there can be no objective science. Each person would occupy himself with the content of his own consciousness, and it would be pointless to dispute about truth. What
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