xīn si
mind
thoughts
inclination
mood
fèi jìn xīn si
to rack one's brains (idiom); to take great pains to think sth through
wā kōng xīn si
to dig for thoughts (idiom); to search everything for an answer
to rack one's brains
yī mén xīn si
to set one's heart on sth (idiom)
huā xīn sī
to think through
to invest effort in thinking about
to consider thoroughly