Halliday & Matthiessen (1999: 206):
Since qualities are assigned to things, they are construed with things as their frame of reference: in the first instance, a quality characterises a thing relative to other things in the same (primary) class. Thus a thick book is not ‘a thick thing’; rather it is a ‘thick book’ as opposed to a ‘slim book’: the scale of ‘thick’ to ‘slim’ is relative to book and a thick book would be much thicker than a thick envelope. This characteristic is particularly noticeable with scalar qualities, which have received particular attention in semantic studies; but it is also, in principle, a feature of taxonomic adjectives — even those construing complex classes. For example, the criteria for assigning ‘wooden’ to a ‘spoon’, a ‘house’ and a ‘carriage’ are fairly different in terms of the actual make-up of these things.