Halliday & Matthiessen (1999: 579):
… the idea clause is projected, as the “content of consciousness”, by the Senser involved in the process of sensing. The content is brought into existence by the sensing process, as actualised through the Senser; and it is construed as being of a higher order of semiotic abstraction than the process of sensing itself (ie it is always at one further remove from the instantial context).