What does it mean to feel sharp?
Today (the day I started writing this, whenever it was) I did not feel sharp. I felt decidedly dull. Other metaphors might apply: slow, versus quick. And thick, versus -- not thick?
What does that even mean?
For me, it felt this way: thinking was hard. It took effort. If reason is a knife, mine was dull, not sharp. If thinking takes time, I was slow, not quick.
Moving a rock takes effort of two kinds. First, there's the physical effort. Your muscles have to push against the rock and the earth. But there's mental effort required as well. When a physical task is easy, "you" can just tell "your body" what to do, and it does it. But when it's hard, you've got to do more. To get "your body" putting out maximum effort, "you" need to "push it" in some way.
No doubt you understand this, absent the scare-quotes for emphasis, because you've done it. To push hard, physically, you have to push hard, mentally. You have to "concentrate yourself." When your muscles tire, you can still get them to do work, but only by pushing them, mentally, with "the will." The amount of will you can direct toward your muscles per unit of time is your "will power."
Thinking, at least some kinds of thinking, involves manipulating mental representations. This requires two different kinds of mental effort. The first is like the effort expended by the muscles: it's the mental work, effort, energy, power required to carry out the manipulation. The second is like to the energy of will in making muscles work. For me, the feeling of "sharpness" or "dullness" correlates to the amount of the second kind of mental energy I need to use to get a given amount of mental work done.
We can actually measure some of the work done by the mind. Mental rotation is the ability to rotate, in the mind, representations of two or three dimensional shapes. The rate at which a person can do the rotation can be measured. Give them two shapes, the the second shape is a rotated version of the first shape, or something else -- perhaps a rotated version of its mirror image. Determining whether the shapes match takes an amount of time that is roughly linear with the number of degrees through which the first must be rotated to match the orientation of the second -- at which time the test subject can decide whether it is a match.
When I'm dull I can't spin those mental images. I can't move ideas from here to there. I can't carry much: when I look up a fact over there so I can use it over here, I drop it before I can get it from where it came to where it's going to.
Intellectually, of course, all these things are illusions. I'm not fast or slow, or dull or sharp. I am able to stop, wake up, and see through the illusion -- for a moment. But the illusion seems to be a ground state, and it seems to take energy to see through it,
Perhaps that, too, is an illusion.