In comparison with classifications of semantic change the problem of their causes appears neglected. Opinions on this point are scattered through a great number of linguistic works and have apparently never -been collected into anything complete. And yet a thorough understanding of the phenomena involved .in semantic change is impossible unless the whys and wherefores become known. This is of primary importance as it may lead eventually to a clearer, interpretation of language development. The vocabulary is the most flexible part of the language and it is precisely its semantic aspect that responds most readily to every change in the human activity in whatever sphere it may happen to take place.
The causes of semantic changes may be grouped under two main headings, linguistic and extra linguistic ones. Of these the first group has suffered much greater neglect in the past and it is not surprising therefore that far less is known of it than of the second. It deals with changes due to the constant interdependence of vocabulary units in language and speech, such as differentiation between synonyms, changes taking place in connection with ellipsis and with fixed contexts, changes resulting from ambiguity in certain contexts, and some other cases.
Semantic change due to the differentiation of synonyms is a gradual change observed in the course of language history, sometimes, but not necessarily, involving the semantic assimilation of loan words. Consider, for example, the words time and tide. They used to be synonyms. Then tide took on its more limited application to the periodically shifting waters, and time alone is used in the general sense.
Another example of semantic change involving synonymic differentiation is the word twist. In OE it was a noun, meaning 'a rope' whereas the verb thrawan (now throw) meant both 'hurl' and 'twist'. Since the appearance in the Middle English of the verb twisten ('twist') the first verb lost this