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expression of certainty, e.g. it can hardly be doubted that…; it goes without saying that …; we have every reason to believe that …;

expression of doubt, e.g. it is doubtful that…; it is hardly acceptable…; there is no reason to think that …;

expression of possibility/impossibility, e.g. it is quite possible that…; we are now in a position to show …; it is easy enough to show…; it would be no less erroneous to believe that …;

expression of necessity/absence of necessity, e.g. it is necessary, therefore to …; we find it necessary to …; it is pointless to state that …;

expression of desirability, usefulness, e.g. it would be most helpful to begin with…; it seems worthwhile to remind that….

The rhetorical patterns of both types are regularly correlated with certain lexical units which serve as their direct markers, e.g. it is common knowledge …; it is a matter of common observation…; we would like to focus on …; this book is intended to show …; as has been explained above …; a further point to be made …; to clarify the point; by this we mean to say that…, etc.
1.9. The category of cohesion in a scientific text. It is generally held that among various means of text cohesion it is lexical means of cohesion that, first and foremost, contribute to the logico-semantic integrity of the text, as they appear to be the most explicitly expressed means of cohesion which facilitate the identification and the establishment of logico-semantic links between different informationally significant parts of a text.
It is also common knowledge that the lexical cohesion of any text finds its expression in:

8. 
   the recurrence of key-words which reflect the most important content points of a text;
   
9. 
   the use of words pertaining to certain lexico-semantic groups united by some common notions;
   
10. 
    the use of words and word-combinations making up certain thematic groups on the basis of common underlying notions;
    
11. 
    the use of words logically associated with the key-words;
    
12. 
    the use of synonyms proper and contextual ones;
    
13. 
    the use of antonyms, both antonyms proper and contextual ones;
    
14. 
    the use of words built up by some common word-building elements (derivatives, compounds, conversion pairs, etc.).
    

But it should be specially stressed that a scientific text is characterized by one more means of lexical cohesion inherent only in this functional variety of language and very important from the point of view of explicit logico-semantic integration of different parts of a scientific text. These linking devices can be referred to as coheremes which function as certain discourse markers to ensure that a scientific text is coherent and cohesive.
Coheremes can be grouped according to their usual functions in a scientific text into: 1. coheremes of immediate linear joining which reflect the progressive development of the author’s reasoning; 2. coheremes of deictic character based on the close association between an antecedent, i.e. the primary designation of something in a text, and a means of its secondary nomination.
As far as coheremes of immediate linear joining are concerned, they can be expressed by certain ‘guide words’ and ‘guide word-groups’ which serve:

1.