Halliday & Matthiessen (1999: 142):
All the modes of perception may be construed either as behaviour or as sensing. One significant grammatical difference is that present behaviour would normally be reported as present-in-present (the present progressive) — What are you doing? I’m watching the last whales of August. — but present sensing would not — I (can) see the whales in the distance. Another one is that only sensing can involve a Phenomenon of the metaphenomenal kind. As long as the ‘phenomenon’ is of the same order of existence as ordinary things, there is no problem with either process type; we can both see and watch macro-phenomena: I saw/watched the last whales leave the bay. But while we can say I saw that he had already eaten we cannot say I watched that he had already eaten, which includes a metaphenomenon. This is the borderline between the mental and the material domains of experience.