Halliday (2008: 81-2):
The system and the text are not two different orders of reality … . There can only be one set of phenomena here, not two. But since there are two possible angles from which to approach and observe them, the same question of point of view arises [as with climate and weather]. Consider the phenomenon of functional variation in language. If we look at this from the “instance” end, we will find in our corpus certain groups of texts that are alike in certain respects; we will be able to recognise something that we call a “text type”. If we look at these same phenomena from the “system” end, we will see this as a sub-system, a recognisable alignment of the meaning-making resources of the system; this is what we refer to as a “register”. Either we see a recurrent pattern of linguistic weather, or we see a definable motif within the linguistic climate. Both of these are possible points of view; but they engender rather different modes of explanation.