Why a Historical Thesaurus
The present work is called a ‘thesaurus’ because it shares with other works (like Roget’s Thesaurus of English Words and Phrases) the feature of wordlists classified according to concept, idea, or nature. However, while it can also be used for the same purposes as Roget its main purpose is entirely different. It is the first historical thesaurus ever produced for any language. Put at its simplest, its purpose is to provide a detailed record of the English vocabulary from the earliest times to the present, with sufficient accompanying information that, for any given period in the past, the user should be able to ascertain the exact state of the vocabulary (that is, the ‘lexical system’) which existed at that time.
This degree of detail is possible only because our main source, the Oxford English Dictionary (OED), is the greatest single repository of facts about vocabulary available for any existing language. It presents citation dates and evidence for all the different meanings of each word, and thus includes, in addition to current meanings, all recorded words that have become obsolete, and all obsolete meanings of those words that still survive. This very full evidence of the OED covers the period from 1150 to the present, but its coverage for the Old English period (700-1150) is more selective. For that period, therefore, it has been necessary to add material from A Thesaurus of Old English.
The purpose of this unique thesaurus, therefore, is to present in a more accessible form the vast amount of information which has hitherto been, as it were, locked away in the alphabetical order imposed by dictionaries – a problem pointed out as long ago as 1943 by the German scholar Walther von Wartburg. Scholars who wished to reconstruct a lexical system of the past for English would accept the list of quasi- and near-synonyms given in Roget as representing the present, and trace back the history of each in the OED. They could perhaps find a few further relevant words in the OED quotations, but otherwise they would have no alternative but to go back to the actual texts of the period in question. If they relied on the evidence of Roget only, they would be omitting all the obsolete words included in the OED, as well as the relevant obsolete meanings of words that still survive. Thus, in the past, a number of monographs on individual words or groups of words have been published, but unless they are the result of very thorough and painstaking research in the texts of their period,  they are criticized because they omitted equally relevant words from that same conceptual system . Electronic access to the OED has improved this situation somewhat, but until the first edition of the Historical Thesaurus was completed there was no systematic conceptual presentation of the development of the vocabulary.
This was a major problem because, in comparison with the detailed and specialized work mentioned above, histories of the English vocabulary have tended to be more general, using criteria like etymology, types of word-formation, or loanwords as focal points for discussion. The obstacle to anything fuller has been the failure to recognize the primacy of meaning, and the analysis of meaning, as the essential tool and criterion for the study of any language; for, in contrast to the closed systems of phonology and grammar, the huge open system of lexis demands nothing less than classification, by meaning, of the whole if one is to begin to understand the parts.
With this purpose in mind, the chronologically ordered lists of meanings presented in the Historical Thesaurus are intended firstly to give a history, with dates of currency, of the words used to express a concept or object, including the losses, additions, and straightforward replacements taking place during that history, and secondly to act as a thesaurus for any given period in the past, so that, for example, anyone wanting to know the range of words available to Shakespeare for a particular meaning can consult the appropriate timespan in the relevant section or sections.
It is the latter function that enables the user to return to the OED and gather fresh information about the character and history of all the words on each individual ‘conceptual map’, including changes of meaning, and redistributions of functions or meanings, especially those ‘sideways shifts’ of meaning which involve replacement by more than one word . In recent decades there has been considerable discussion about reasons for changes in the vocabulary. Some of the explanations advanced strike us as obvious, e.g. foreign invasion, the wealth of Latin loanwords at the time of the Renaissance, new inventions, or the simple replacement of one artefact by another. But other changes, and the reasons for them, are much more controversial. For example, opinions vary greatly on the relevance, as factors in vocabulary change, of features like homophony, near-homophony, phonetic and/or phonaesthetic suitability, and polysemy . We aim through our presentation here of the Thesaurus’ chronological lists (including obsolescences and therefore the choices of past speakers and writers) to provide today’s scholars and critics with more of the evidence they need to answer such questions. We also aim with Historical Thesaurus data to continue to offer fresh perspectives to scholars in other fields, for example anthropologists, historians, and others interested in cultural developments.
In addition to opening up the study of the successive and changing lexical systems of the past, our work on the classification of meaning produces a whole new vertical perspective on each individual concept. Following the list of successive forms and their dates for the main concept, hierarchically organized sub-sections enable the user to compare the dates of closely related items. The results, especially where they include such tree-like structures, each with its own complete set of dates, can contribute substantially to determining the historical status of a concept with an observable past history. Such comparative procedures can be applied on the abstract side to the history of ideas, and on the more concrete to a large proportion of the remainder of the contents of the Historical Thesaurus, ranging from broad subjects like cultural and social history to others more specialized, such as military and domestic history.
 Jane Roberts & Christian Kay with Lynne Grundy, A Thesaurus of Old English. (King’s College London Medieval Studies XI), 1995. 2nd edn, Amsterdam: Rodopi, 2000. Online version 2005.
 Einführung in Problematik und Methodik der Sprachwissenschaft. Tübingen, 1943. Revised edition with the collaboration of Stephen Ullmann, translated from the French edition by Joyce M. H. Reid, Oxford: Blackwell, 1969.
 E.g. A. Rudskoger, Fair, Foul, Nice and Proper: A Contribution to the Study of Polysemy. (Gothenburg Studies in English I.) Stockholm: Almqvist & Wiksell, 1952.
 E.g. A. Rynell, The Rivalry of Scandinavian and Native Synonyms in Middle English. (Lund Studies in English XIII.) Lund: Gleerup, 1948.
 For some results of such procedures, see Jeremy Smith, An Historical Study of English: Function, Form, and Change. London: Routledge, 1996, 135-139.
 See further: E. R. Williams, The Conflict of Homonyms in English. (Yale Studies in English, 100.) Yale University Press: New Haven, Conn, 1944; R. J. Menner, "Multiple Meaning and Change of Meaning in English". Language 21, 1945, 59-76; M. L. Samuels, Linguistic Evolution. Cambridge: Cambridge University Press, 1972, 67-77; Jeremy Smith, op. cit., 120-123, 135-139; Dirk Geeraerts, Diachronic Prototype Semantics. Oxford: Oxford University Press, 1997; Philip Durkin, The Oxford Guide to Etymology. Oxford: Oxford University Press, 2009.