Mental dullness also has coarse and subtle forms. Coarse dullness occurs when the mind is disabled due to worries and obsessive concerns. The mind is sluggish and unwieldy due to its preoccupation with itself. Your attention may be placed squarely on an object, but there will be no clarity or distinctiveness. The subtle form of dullness is so difficult to understand and recognize, that it is often mistaken for perfect samadhi. Such a mistake firmly blocks further progress.
Coarse dullness is easily recognized but is difficult to overcome, while subtle dullness is difficult to recognize, but easily overcome. Subtle dullness is like an enemy who pretends to be a friend. One has both memory and clarity of mind, but the full force of attention is lacking, and the mind will lack complete power. This is something a person must recognize for himself.
The remedy for both agitation and dullness is mental alertness. Alertness is a portion of the mind that can stand back and observe the workings of the mind, and then take appropriate action. It observes the progress of thought, and checks backward slipping. When one recognizes agitation and dullness one must quickly recall one's deep need of samadhi and regain confidence and certainty.