Prove that any 6 persons inside existence three persons know each other or exist three humans do not know each other "graph theory, Drawer principle"

The 6/7-month 1958 issue of the American Journal of Mathematics has the following title:
"Prove that at any 6 people's rally, or 3 people have known each other before, or three people have not met each other before."
This problem can be easily and clearly demonstrated in the following ways:
On the plane, use 6 points A, B, C, D, E, F, respectively, to represent any 6 persons participating in the rally. If two people knew each other before, they would be a red line on behalf of their two points, or a blue line. Consider the 5 connection between point A and the remaining points Ab,ac,..., AF, They have no more than 2 colors. According to the principle of the drawer can be found at least 3 lines of the same color, it may be set Ab,ac,ad red. If BC,BD, there is one in the CD 3 line (may be set to BC) is also red, then the triangle ABC is a red triangle, a, B, C represents the 3 people who used to know each other: if the BC, BD, CD 3 lines are all blue, then the triangle BCD is a blue triangle, B, C, D for the 3 people have not known each other. In either case, it is consistent with the conclusion of the question.
The six-person assembly problem is one of the simplest special cases of the famous Ramsey theorem in combinatorial mathematics, and the proof thought of this simple problem can be used to draw some further conclusions. These conclusions constitute the important content of combinatorial mathematics-----Ramsey theory. From the six-person rally, Once again we saw the application of the drawer principle.

Prove that any 6 persons inside existence three persons know each other or exist three humans do not know each other "graph theory, Drawer principle"