Genuine enquiry is a legitimate path to knowledge but sceptical or cynical doubting destroys faith and confidence. Why do we doubt? How to deal with the habit of this corrosive doubting?
Doubt is not a sport to indulge in with impunity; it is a poison which drop by drop corrodes the soul.
We must decide to get rid of all doubts, they are among the worst enemies of our progress.
[CWM2, 14:244]
Doubt is the best arm used by the ego to protect itself from extinction.
[CWM2, 14:199]

It is easier to Doubt then to know

… it is easier to doubt than to know, the human mind is accustomed to doubt everything; that
is its first movement, and of course that is why it knows nothing.
[CWM2, 9:237]
In fact, so long as there is any doubt or hesitation, so long as one asks oneself the question of whether one has or hasn’t realised this eternal soul in oneself, it proves that the true contact has not taken place. For, when the phenomenon occurs, it brings with it an inexpressible something, so new and so definitive, that doubt and questioning are no longer possible.
[CWM2, 9:336]
This is what is at the root of all the misunderstandings and reservations. You already know, and I mention it only to remind you, that an experiment made in a spirit of reserve and doubt is not an experiment, and that outer circumstances will always conspire to justify these doubts, and this for a reason which is very easy to understand: doubt veils the consciousness and the subconscious sincerity, and into the action some small factors creep in which may seem unimportant, but which are just sufficient to alter all the factors of the problem and to bring about the result that one had anticipated by doubting.
[CWM2, 16:41]
One knows that something is true but still doubts. Why does one doubt the truth?
The usual answer, it is because one is foolish! (Laughter)
But the truth is that the physical mind is truly completely stupid! You can prove it very easily. It is constructed probably as a kind of control, and in order to make sure that things are done as they ought to be. I think that this is its normal work…. But it has made it a habit to doubt everything.
I think I have already told you about the small experiment I made one day. I removed my control and left the control to the physical mind—it is the physical mind which doubts. So I made the following experiment: I went into a room, then came out of the room and closed the door. I had decided to close the door; and when I came to another room, this mind, the material mind, the physical mind, you see, said, “Are you sure you have locked the door?” Now, I did not control, you know… I said, “Very well, I obey it!” I went back to see. I observed that the door was closed. I came back. As soon as I couldn’t see the door any longer, it told me, “Have you verified properly?” So I went back again…. And this went on till I decided: “Come now, that’s enough, isn’t it? Closed or not, I am not going back any more to see!” This could have gone on the whole day. It is made like that. It stops being like that only when a higher mind, the rational mind tells it, “Keep quiet!” Otherwise it goes on indefinitely…. So, if by ill-luck you are centred there, in this mind, even the things you know higher up as quite true, even things of which you have a physical proof—like that of the closed door, it doubts, it will doubt, because it is built of doubt. It will always say, “Are you quite sure this is true?… Isn’t it an idea of yours?… You don’t suppose it is like that?” And it will go on until one teaches it to keep quiet and be silent.
[CWM2, 6:224-25]

Is Doubt a sign of superiority?

Men believe that doubt is a sign of superiority, whereas it is really a sign of inferiority.
Scepticism and doubt are two of the greatest obstacles to progress; they add presumptuousness to ignorance.
[CWM2, 10:27]
The enemy of faith is doubt and yet doubt too is a utility and necessity, because man in his ignorance and in his progressive labour towards knowledge needs to be visited by doubt, otherwise he would remain obstinate in an ignorant belief and limited knowledge and unable to escape from his errors. This utility and necessity of doubt does not altogether disappear when we enter on the path of Yoga.
[SABCL, 21:744]

Some practical suggestions

…to help at the beginning, one can take as a guiding rule that all that brings with it or creates peace, faith, joy, harmony, wideness, unity and ascending growth comes from the Truth; while all that carries with it restlessness, doubt, scepticism, sorrow, discord, selfish narrowness, inertia, discouragement and despair comes straight from the falsehood.
[CWM2, 12:302]
In the human mind there is a morbid and deplorable habit of doubt, argument, scepticism. This is where human effort must be put in: the refusal to admit them, the refusal to listen to them and still more the refusal to follow them. No game is more dangerous than playing mentally with doubt and scepticism. They are not only enemies, they are terrible pitfalls, and once one falls into
them, it becomes tremendously difficult to pull oneself out.
[CWM2, 9:351]

How to get rid of Doubt

I have started writing about doubt, but even in doing so I am afflicted by the “doubt” whether any amount of writing or of anything else can ever persuade the eternal doubt in man which is the penalty of his native ignorance. In the first place, to write adequately would mean anything from 60 to 600 pages, but not even 6000 convincing pages would convince doubt. For doubt exists for its own sake; its very function is to doubt always and, even when convinced, to go on doubting still; it is only to persuade its entertainer to give it board and lodging that it pretends to be an honest truth-seeker. This is a lesson I have learnt from the experience both of my own mind and of the minds of others; the only way to get rid of doubt is to take discrimination as one’s detector of truth and falsehood and under its guard to open the door freely and courageously to experience.
[SABCL, 22:167-68]
As to doubts and argumentative answers to them, I have long given up the practice as I found it perfectly useless. Yoga is not a field for intellectual argument or dissertation. It is not by the exercise of the logical or the debating mind that one can arrive at a true understanding of yoga or follow it. A doubting spirit, “honest doubt” and the claim that the intellect shall be satisfied and be made the judge on every point is all very well in the field of mental action outside. But yoga is not a mental field, the consciousness which has to be established is not a mental, logical or debating consciousness – it is even laid down by yoga that unless and until the mind is stilled, including the intellectual or logical mind, and opens itself in quietude or silence to a higher and deeper consciousness, vision and knowledge, sadhana cannot reach its goal. For the same reason an unquestioning openness to the Guru is demanded in the Indian spiritual tradition; as for blame, criticism and attack on the Guru, it was considered reprehensible and the surest possible obstacle to sadhana.
If the spirit of doubt could be overcome by meeting it with arguments, there might be something in the demand for its removal by satisfaction through logic. But the spirit of doubt doubts for its own sake, for the sake of doubt; it simply uses the mind as its instrument for its particular dharma, and this not the least when that mind thinks it is seeking sincerely for a solution of its honest and irrepressible doubts. Mental positions always differ, moreover, and it is well-known that people can argue for ever without one convincing the other. To go on perpetually answering persistent and always recurring doubts such as for long have filled this Ashram and obstructed the sadhana, is merely to frustrate the aim of the yoga and go against its central principle with no spiritual or other gain whatever. If anybody gets over his fundamental doubts, it is by the growth of the psychic in him or by an enlargement of his consciousness, not otherwise. Questions which arise from the spirit of enquiry, not aggressive or self-assertive, but as a part of a hunger for knowledge can be answered, but the “spirit of doubt” is insatiable and unappeasable.
[SABCL, 22:162-63]
X told me that it is very good to read Sri Aurobindo’s messages on doubt. “It is not at all necessary to read about doubt,” I answered him. “At this stage I don’t think it is possible to be free from all doubt,” he said.
It is not only quite possible to be free of all doubt, it is an absolutely indispensable condition; but before being able to state with assurance that one is free from doubt, one should wait for a few months at least, in order to make sure.
In any case this kind of discussion is quite useless—it does not help to overcome doubt.
[CWM2, 17:50-51]