Halliday & Matthiessen (2004: 461):
With the ‘mental’ reporting of ideas, there is an important distinction between propositions and proposals, deriving from their fundamental nature as different forms of semiotic exchange. Whereas propositions, which are exchanges of information, are projected mentally by processes of cognition — thinking, knowing, understanding, wondering, etc — proposals, which are exchanges of goods-&-services, are projected mentally by processes of desire … . Thus, while propositions are thought, proposals are hoped.