Halliday & Matthiessen (1999: 167-8):
Representatives of a class can be impacted (regardless of whether they are specific or non-specific at the point in the discourse at which they occur); but it is harder to impact the general class itself. Consequently, if the Goal is a general class rather than a set of specifiable representatives, it has a lower degree of participanthood. This appears iconically in the grammar in the limiting case of a clause where the Goal is simply that class of phenomenon that can serve as Goal of that particular type of figure: the grammar allows us to select 'goal-intransitive', which means that the Goal is simply not specified — for example:¹³he drinks heavily [alcohol]he eats all the time [food]they've gone to the hills to hunt [game]he buys and sells [any commodity]Such examples are typically either habitual (the process unfolds repeatedly) or durational (the process unfolds over time): this generalisation across time correlates with the generalisation across potential participants — both are ways of generalising from experience. In some special cases the generalisation of the Goal across a class of entities is shown by treating it as a mass, dispensing with the plural marker:They often shoot duck during the winter months.Such a Goal may even be incorporated into the Process, as in he is baby-sitting (and even who's baby-sitting me this evening?)', this is a restricted option with figures realised as ranking clauses, but not uncommon where the figure is used to qualify an element and is realised by a pre-modifying clause, e.g. a fun-loving colleague, a wood-burning stove.
¹³ Here, because of its generality, the Goal is predictable experientially. The Goal may of course be predictable textually, which is the reverse case: so specific at that point in the discourse that it can be anaphoricaliy presumed. Typically in such cases an explicit pronoun is used to refer back; but it can be omitted in certain registers, especially instructional ones such as recipes: when all the pancakes are made, garnish the dish and serve _ with cheese and egg sauce.