All distress begins from thoughts. Thoughts begin to wander; they arouse suspicion, mistrust, and condemnation, slacken the work, and produce discord and disobedience. Therefore one must keep watch and not allow them to wander at their own will. Those who control their thoughts have everything in order. The ascetic practice of keeping thoughts under control is called mental vigilance.
One cannot live without thinking, because the mind is given to us for this purpose. But we may have good, bad or idle thoughts. Keeping vigilance over our mind consists in casting away bad or idle thoughts and keeping only the good ones which are in accord with the will of God and His holy commandments, and doing this in every action or situation which may occur.
The measure of good thoughts is the word of God. In every case, keep your thoughts in accordance with Holy Scripture. If you do so, you will have order in your mind; and if your mind is in order, so will be your actions, and you will have harmony among yourselves.
~St Theophan the Recluse