There is a land where every inhabitant is either a day-knight or a night-knight. Day-knights tell the truth in the day and lie at night. Night-knights lie in the day and tell the truth at night. One inhabitant once said, "It is day." Is the inhabitant a day-knight or a night-knight?
Usually, in similar puzzles knights never lie, so it would be wiser to use a terminology tailored to fit this peculiar land
e.g. D-knave & N-knave.
Addressing the puzzle per se we can check 4 cases:
c1: The inhabitant is a day- knight and it is day
- He'd say "it is day".
c2: The inhabitant is a day- knight and it is night
- he would say "it is day".
c3: The inhabitant is a night- knight and it is day
- He'd say "it is night".
c4: The inhabitant is a night- knight and it is night
- he would say "it is night".
Ergo, the speaker was a day-knight
night and day
