Necessary and sufficient condition: The Friend

To consider the person a friend it is necessary and sufficient to:

1. Know the person

2. Like the person.

3. Trust the person.

Axiom 1. Condition #1 has to be mutual. Otherwise you are probably a stalker.

Axiom 2. Condition #2 may be one-way. In the worst case the friendship is non-reciprocative.

Axiom 3. Condition #3 may be one-way. In the worst case you can still cry drunkenly on his shoulder but don't expect a honest advise.

Without #1 Warren Buffett could be your friend.

Without #2 your CRA agent could be your friend.

Without #3 your favourite lap dancer can be your friend.