From Feeling Good: The New Mood Therapy by David Burns
You take your emotions as evidence for the truth. Your logic: "I feel like a dud, therefore I am a dud." This kind of reasoning is misleading because your feelings reflect your thoughts and beliefs. If they are distorted--as is quite often the case--your emotions will have no validity. Examples of emotional reasoning include "I feel guilty. Therefore, I must have done something bad"; "I feel overwhelmed and hopeless. Therefore, my problems must be impossible to solve"; "I feel inadequate. Therefore, I must be a worthless person"; "I'm not in the mood to do anything. Therefore, I might as well just lie in bed"; or "I'm mad at you. This proves that you've been acting rotten and trying to take advantage of me."
Emotional reasoning plays a role in nearly all your depressions. Because things feel so negative to you, you assume they truly are. It doesn't occur to you to challenge the validity of the perceptions that create your feelings.
One usual side effect of emotional reasoning is procrastination. You avoid cleaning up your desk because you tell yourself, "I feel so lousy when I think about that messy desk, cleaning it will be impossible." Six months later you finally give yourself a little push and do it. It turns out to be quite gratifying and not so tough at all. You were fooling yourself all along because you are in the habit of letting your negative feelings guide the way you act.