Right now you are probably in a position to perceive the weather outside. Take a closer look. Now close your eyes or turn them elsewhere and merely think about the weather outside. Entertain various propositions about it. Now reflect on the difference between those modes of being conscious of the weather. I think it's obvious, first, that there is a pronounced phenomenological difference between the two experiences. To see the weather conditions, to determine perceptually that it is sunny, for instance, requires that you undergo one of a certain range of perceptual experiences. You can merely think about the weather, however, no matter what you are perceiving. You can do so whether your eyes are open or closed, whether you perceive an open book or the contents of your refrigerator. The difference in epistemic worth of the two experiences is equally profound. Merely thinking that it is sunny, say, does not give one a reason to believe that it is. Seeing that it is sunny, on the other hand, provides an excellent reason to believe that it is.

The distinction between merely thinking about something emptily and having it bodily present in perception is arguably the most important distinction in both the theory of intentionality and the theory of knowledge. Despite assurances to the contrary by a few influential philosophers, there can be no serious doubt that perceptual experiences can and do provide warrant or justification for beliefs.