"The House of Israel is composed of 600,000 souls, corresponding to which the world is composed of 600,000 segments.
This reckoning is not so simple, because the nation of Israel has always consisted of more than 600,000 people.
These 600,000 particular souls are roots, and each root subdivides into 600,000 sparks,
The soul of a Jew at a particular time and place is not generally speaking one of the 600,000 original souls but only a part of it, a spark.
This division of the root souls explains why on occasion two people who apparently have nothing in common—family ties, educational background, or shared viewpoints—feel a bond between them.
The underlying reason for this mysterious closeness is the essence of their inner souls.
They feel close to each other because their souls are close; they feel suited to each other because they are no more than two branches of one soul.
The fact that they are not necessarily similar is because the root soul does not always branch out in a symmetrical fashion.
One person may be more intellectual and another more emotional.
Yet they will feel an inexplicable connection, for they are fragments of one soul."
--Rabbi Adin Steinsaltz
From Understanding the Tanya by Rabbi Adin Steinsaltz
