All 3 Uses of

sober
in
Zen and the Art of Motorcycle Maintenance

From that agony of bare existence to modern life can be soberly described only as upward progress, and the sole agent for this progress is quite clearly reason itself.†
Part 2 *

soberly = in a serious manner

It was hardly guessed by anyone that within a few decades there would be no more absolute space, absolute time, absolute substance or even absolute magnitude; that classical physics, the scientific rock of ages, would become "approximate"; that the soberest and most respected of astronomers would be telling mankind that if it looked long enough through a telescope powerful enough, what it would see was the back of its own head!†
Part 3 *

Definitions:

(1) (sober as in: Talk to me when your sober.)
not under the influence of alcohol
(2) (sober up as in: I need to sober up.)
to become less drunk or intoxicated
(3) (sobering as in: a sobering thought)
serious or calm (not silly or excited); or making one serious or less excited
(4) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

More rarely, sober can refer to one who is rational rather than under the influence of extreme emotion. Also more rarely, it can reference that which is practical rather than fanciful -- such as "a more sober plan."