Halliday & Matthiessen (1999: 613):
… because the protolanguage sets limits on both dimensions of meaning. You can converse in it, but you cannot build up a dialogue: that is, it allows exchange of meaning, but it precludes any form of interpersonal dynamic, in which meanings expand on the basis of what went before. You can point with it, but you cannot refer: that is, it allows focus on an object, but it precludes any form of ideational systematic, in which phenomena are construed as configurations and in taxonomies.