Rudolf Steiner: "I have said to you again and again that control of thought is what is necessary. And control of thought consists not merely in that a thought is clear; but rather that every thought must have a "control" thought. One should never think or speak a thought without applying the control thought that belongs to it. Human beings can work wonders if they do not allow themselves to conceive only one-sided thoughts."
Source: Concerning the Astral World and Devachan, p. 166