Halliday & Matthiessen (1999: 233):
There are two other characteristics of lexical metaphors which are also central to metaphor in its grammatical sense. The first is syntagmatic: lexical metaphors tend to occur in regular clusters, which we shall refer to here as "syndromes"; for example, the metaphor congregation => flock forms a syndrome together with religious official => shepherd, group of believers => fold and so on.The second is paradigmatic: lexical metaphors typically involve a shift towards the concrete, a move in the direction of "objectifying" ('making like an object', not 'making objective'), as the same examples show.