Rabbi Hillel

That which is unpleasant to you, do not to your neighbor.  That is the whole law and the rest but it's exposition. 

What is hateful to thee, do not unto thy fellowman;
this is the whole Law. The rest is but commentary.

If I am not for myself, who is for me? And if I am only for myself, what am I? If not now, when?

Moreover he saw a skull floating on the surface of the water and he said unto it: Because you drowned others they drowned you; and those that drowned you will eventually be drowned.

If Not Now, When?

In a place where there are no humans One must strive to be human

Do not unto others that which is hateful unto thee.

If I am not for myself, who is for me? And if I am only for myself, what am I? If not now, when?

Judge not thy friend until thou standest in his place.

What you yourself hate, don't do to your neighbor. This is the whole law; the rest is commentary. Go and study.