In terms of what we'll call "internal" sensations, by which I mean mental movements, thoughts, conceptualizing, and anything of a not-solely-physical-sense-based variety, the selection of what can be attended to is pretty much endless. What's useful is not to get involved in the content, simply notice them and see how they appear, fade and vanish by themselves to be replaced by something else. With further practice, you can also dismantle and investigate these states in the same way as you would with any other sensation. It's all fair game, the point is (if such a thing can even be said!) that you're attentive to what's happening in your immediate experience right now. Once you can be attentive effortlessly, practice being