Dictionary Definition: An Opinion

There are a number of traditional assumptions about language which have new been shown to be mistaken; in particular, criticism has come from Russell and Saussure. Their arguments are well-documented elsewhere, and the reader is referred to their own works for a detailed argument. But what concerns me is the mistaken outlook on language against which their criticism was directed, and the implications of this on linguistic competence, particularly as regards the usage of dictionaries.
 
The false assumptions that have been made about language are well summarised by Antony Thiselton:
 
1   that the word, rather than the sentence or speech-act, constitutes the basic unit of meaning to be investigated;
 
2   that questions about etymology somehow relate to the real or "basic" meaning of a word;
 
3   that language has a relation to the world which is other than conventional, and that its "rules" may therefore be prescriptive rather than merely descriptive.
 
It seems that these assumptions are still very widespread. To take but a simple example: it is common practice to take the dictionary definition as the ultimate appeal upon questions of meaning.
 
Inevitably, this leads to an ignorance of the fact that "language functions on the basis of convention, and is not in fact 'reality"' (Thiselton). In other words, this approach fosters the pernicious idea that there is something absolute about words; that there exists an essential or "real" meaning of a word, and we may find this by appealing to the dictionary.
 
The dictionary, however, mainly provides synchronic word studies of conventional meanings, although it may contain a proportion of diachronic word studies in so far as the past meaning of words is still important to understanding in contemporary culture.
 
But all meanings which it gives are conventionally accepted social ones; they only describe usage; they cannot prescribe usage. It can be seen that if dictionary definition was prescriptive, there would be no distinction between diachronic and synchronic studies: words would have the same set of meanings today as over a period of historical change. But words do change in meaning with the passage of time, so this is clearly false.
 
Obviously it is useful to understand the meaning of an unknown word, but it must be understood contextually. The dictionary may provide a set of possible meanings of a given word, and thereby function as a guide to synchronic meaning - but it is we who must decide which meaning is appropriate to the given context.
 
It should be explained to the public that the dictionary is no more than a guide to the meaning of words, else, by default, we run the risk of misleading them into the acceptance of some form of methodological essentialism.
 
There seems to be widespread ignorance of these limitations on dictionary definitions, and I cannot help wondering whether it might be due, in part, to a defect within our educational system. It is all very well to instruct small children to find out meanings by looking in a dictionary, but where in later schooling are they instructed otherwise?