http://cities.tumblr.com/post/43582201/some-people-are-toxic-avoid-them

There was in the sixties a man named Fritz Perls who was a gestalt therapist. Gestalt therapy derives from art history, it proposes you must understand the ‘whole’ before you can understand the details. What you have to look at is the entire culture, the entire family and community and so on. Perls proposed that in all relationships people could be either toxic or nourishing towards one another. It is not necessarily true that the same person will be toxic or nourishing in every relationship, but the combination of any two people in a relationship produces toxic or nourishing consequences. And the important thing that I can tell you is that there is a test to determine whether someone is toxic or nourishing in your relationship with them. Here is the test: You have spent some time with this person, either you have a drink or go for dinner or you go to a ball game. It doesn’t matter very much but at the end of that time you observe whether you are more energised or less energised. Whether you are tired or whether you are exhilarated. If you are more tired then you have been poisoned. If you have more energy you have been nourished. The test is almost infallible and I suggest that you use it for the rest of your life.

— Milton Glaser, from this