" In view of the dangers of [external] reality, the ego is obliged to guard against certain instinctual impulses in the id and to treat them as dangers. But it cannot protect itself from internal instinctual dangers as effectively as it can from some piece of reality that is not part of itself. Intimately bound up with the id as it is, it can only fend off an instinctual danger by restricting its own organization and by acquiescing in the formation of symptoms in exchange for having impaired the instinct. If the rejected instinct renews its attack, the ego is overtaken by all those difficulties which are known to us as neurotic ailments. "
― Sigmund Freud , Inhibitions, Symptoms and Anxiety