The distinction between modal words and adverbs is, as we saw in our general survey of parts of speech, based on two criteria: (1) their meaning: modal words express the speaker's view concerning the reality of the action expressed in the sentence, (2) their syntactical function: they are not adverbial modifiers but parentheses, whether we take a parenthesis to be a special part of the sentence or whether we say that it stands outside its structure. The latter problem is one that we will discuss in Syntax.1 We must emphasise that this view is far from being the only one possible: one might argue that the meaning of an adverb as a part of speech might be described in such a way as to include what we call modal words, and to mention the function of parenthesis among the syntactical functions of adverbs. Where clear objective morphological criteria fail there will always be room for different interpretations. We will not argue this point any further but start on the assumption that modal words do constitute a separate part of speech. Modal words have been variously classified into groups according to their meaning: those expressing certainty, such as certainly, surely, undoubtedly; those expressing doubt, such as perhaps, maybe, possibly, etc. The number of types varies greatly with different authors. We need not go into this question here, as this is a lexicological, rather than a grammatical, problem. From the grammatical viewpoint it is sufficient to state that all modal words express some kind of attitude of the speaker concerning the reality of the action expressed in the sentence. In the vast majority of cases the modal word indicates the speaker's attitude towards the whole thought expressed in the sentence (or clause), e.g. Look, there are those doves again. The one is really quite a bright red, isn't it? (R. WEST) She is a delicate little thing, perhaps nobody but me knows how delicate. (LAWRENCE) If the modal word in each of the sentences is eliminated the whole thought will lose the modal colouring imparted to it by the modal word, and will appear to be stated as a fact, without any specific mention of the speaker's attitude. However, occasionally a modal word may refer to some one word or phrase only, and have no connection with the rest of the sentence. It may, for example, refer to a secondary part of the sentence, as in the following example: No one expected his arrival, except Rose presumably. (LINKLATER) The use of modal words depends to a great extent on the type of the sentence. This will be discussed in Chapter XXIV A modal word can also make up a sentence by itself. This happens when it is used to answer a general question, that is, a question admitting of a yes- or no-answer. Certainly, perhaps, maybe, etc. may be used in this way. In these cases, then, modal words are the main part of the sentence. This brings them close together with the sentence words yes and no. ' However, they differ from the sentence words in that the modal words can also be used as parentheses in a sentence. Thus, the question, Are you coming? may equally be answered, Certainly I am, or Certainly. The sentence words yes and no cannot be used as parentheses. Whether the answer is Yes, or Yes, I am, the yes is a sentence in both variants. It might be possible to argue that if the answer to the question Are you coming? is Certainly, the word certainly is a parenthesis, and the rest of the answer, / am, is "understood". While such a view cannot be disproved, it seems unnatural and far-fetched, and we will prefer the view that Certainly in this case is a sentence. The problem of modal words is connected with the very difficult problem of modality as a whole. This has been treated repeatedly by various scholars both with reference to English and to Russian and in a wider context of general linguistics as well.2 We will not investigate here all the aspects of the problem. We will only mention that there are various means of expressing modality — modal words, modal verbs (can, must, etc.) and the category of mood. Since two of them or even all three may be used simultaneously, it is evident that there may be several layers of modality in a sentence. A great variety of combinations is possible here.