lost in thought
mind
thoughtless
live rent-free in someone's head
coconsciousness
bemused
preoccupied
lost
absorption
engrossment
preoccupancy
preoccupation
distracted
space cadet
abstracted
scatty
absentminded
absent
make a better door than a window
get ahead of oneself
autopilot
in another world
in one's own little world
in a world of one's own
in one's own world
puzzleheaded
narrow-minded
circuit
tunnel vision
absent-minded professor
challenging
thought-provoking
woolgathering
wander
blue-sky thinking
present
presence
think over
in idea
determination
conclusion
count one's blessings
decision
musing
extranoematic
intracognition
obsessed
theory of mind
intrapersonal
manodanda