Darkness and light

A Rabbi asked his students, How can we determine the hour of dawn, when the night ends and the day begins?

One of his students suggested, When from a distance you can distinguish between a dog and a sheep?
No, was the answer from the Rabbi.
Is it when one man can distinguish between a fig tree and a grape vine? asked a second student.
No, the Rabbi said.

Please tell us the answer, then, said the students.
It is, then, said the wise teacher, when you can look into the face of a stranger and you have enough light within you to recognize them as your brothers and sisters.
Up until then it is night, and darkness is still with us.