Halliday & Matthiessen (1999: 520):
The changes that constitute our experience are not all independent of one another. On the contrary; our experience is that one thing leads to another, and there is in principle no limit to an experiential chain. But the exact nature of the relationship may vary from one transition to another; so the grammar construes the relationship between processes dyadically, in the form of a nexus between a pair of clauses. The first process may have a second process related to it, by a relationship such as sequence in time, or cause and effect; this in turn may have another one related to it, either by the same relationship or by a different one — in either case, the relationship is construed as holding between the members of a pair. So the logical system, within the ideational metafunction, engenders a different kind of linguistic structure from that of the experiential system. In the logical world, the parts are not constituents of an organic configuration, like the process, participants and circumstances of the clause. They are elements standing to each other in a potentially iterative relationship; and each element represents an entire process.