As we saw, a very wide range of semantic relationship is encoded through the clause complex. But in order to construct discourse we need to be able to establish additional relations within the text that are not subject to these limitations; relations that may involve elements of any extent, both smaller and larger than clauses, from single words to lengthy passages of text; and that may hold across gaps of any extent, both within the clause and beyond it, without regard to the nature of whatever intervenes. This cannot be achieved by grammatical structure; it depends on a resource of a rather different kind. These non – structural resources for discourses are what are referred to by the term COHESION.
- Halliday (1985), a pag.288

Cohesion is, of course, a process, because discourse itself is a process [...] it is natural to talk about cohesion as a relation between entities, in the same way that we talk about grammatical structure [...] The organization of text is semantic rather than formal [...] We shall represent cohesive relations simply by additions to the structural notation. But it is important to be able to think of text dynamically, as an ongoing process of meaning; and of textual cohesion as an aspect of this process, whereby the flow of meaning is channelled along the speaker’s purposive courses instead of spilling out aimlessly in every possible direction.
- Halliday (1985), a pag.290