A doubt is a grey area. Grey is something which is neither white nor black. Now, how to solve a doubt? Accept a doubt as either black or white. See your doubt as white and there is no doubt. See the doubt as black and accept it. Either way, you accept it and move on.
See someone as either honest or dishonest and accept him/her. Then your mind is quiet. Then you are not in the grey area of doubt. Have conviction: “He is dishonest and yet he is still part of me. I accept him as he is.” Doubt is an unstable state with footing neither on this shore nor that shore. From there tension arises.
Doubt is a state of mind. Do you know when this state of mind is present? It is when we are body-conscious or body-bound, and not aware of the life within us. Doubt is that state of mind where it’s not fully alive. Answers cannot satisfy doubt. Hearing and seeing cannot eliminate doubt.
Have you noticed that you usually doubt only the things that are positive in your life? You don’t doubt negative things. You doubt a person’s honesty, and you believe in his dishonesty. When someone is angry with you, you have no doubt about his anger. But when someone says he loves you, a doubt creeps in: Does he really love me? When you are depressed, do you ever think: Am I really depressed? No, you take your depression as a fact. Yet when you are happy, you doubt: Am I really happy; Is this really what I wanted?
See this tendency to doubt the positive things in your life. Put doubt in its proper place and doubt the doubts. Doubt the negative and put your trust more in the positive.
When the prāna is low, you feel unfulfilled and your mind is filled with doubts. When the life force in you is dull, the wick in the lamp is going down, what do you do? On the day you do prāṇāyāma or Sudarshan Kriyā, you are so alive. Have you noticed it? Every cell in your body is alive, and then there is no doubt in the mind. You feel so wonderful. There is no split in your consciousness. Doubt causes this split in our consciousness. A portion is awake and another major portion is asleep — the unfulfilled state of consciousness. And when the prāna is full, you are so fulfilled. So when you have doubts, do more sādhanā or spiritual practices. You'll immediately see the shift in your consciousness.