This passage on introspection from Lewis is one of the very best observations in the entire book. It’s worth reading it twice. I’ll have a ton more to say about this at some point.
In introspection we try to look “inside ourselves” and see what is going on. But nearly everything that was going on a moment before is stopped by the very act of our turning to look at it. Unfortuntely this does not mean that instrospection finds noting. On the contrary, it finds precisely what is left behind by the suspension of all our normal activities; and what is left behind is mainly mental images and physical sensations. The great error is to mistake this mere sediment or track or byproduct for the activities themselves. That is how men may come to believe that thought is only unspoken words, or the appreciation of poetry only a collection of mental pictures, when these in reality are what the thought or the appreciation, when interrupted, leave behind – like the swell at sea, working after the wind has dropped.
-C.S. Lewis, Surprised by Joy, p.219