the clue on the tip of the tongue
brooding uncomfortably for a moment on why he gets off on the secrecy of getting high in secret more than on the getting high itself, possibly. He always gets the feeling there’s some clue to it on the tip of his tongue, some mute and inaccessible part of the cortex, and then he always feels vaguely sick, scanning for it.
Context
The secrecy-attachment returns, and with it the exact texture of Hal's blindness to himself: the answer to why the hiding matters more than the high sits on the tip of his tongue, in some mute inaccessible fold of the mind, and every attempt to reach it makes him faintly sick. He can feel the shape of the thing and cannot say it, which is the whole diagnosis he keeps not making.