Some people are concerned when they read that doubt is a hindrance on the path. They think this means Buddhists should take things wholly on blind faith, and that questioning is wrong. If we are sceptical, we do not believe everything we hear. This is a good approach. And as the quotes below show, scepticism is not unwise in Buddhism (nor in everyday life). However, what is translated as sceptical doubt is a barrier to any development. It is unwise to believe everything we are told. And, equally unwise to doubt everything we are told − which is sceptical doubt.
"Like testing gold , upon being scorched, cut and rubbed,
My word is to be adopted by monastics and scholars
Upon analyzing it well,
Not out of respect [for me]"
Elsewhere, Gotama taught (To a sceptical group of people called the Kalamas):
Come Kalamas. Do not believe something because you hear it frequently; nor because people have always done it that way; nor upon rumour; nor upon what is in a scripture; nor upon incomplete evidence; nor upon what is obvious; nor upon clever, but wrong reasoning; nor upon something that accords with your pet ideas; nor upon another's impressive ability; nor because your teacher says so. Kalamas, when you yourselves know: "These things are bad; these things are blameable; these things are censured by the wise then do not follow these principles. They will not help you. Abandon them!
But when, Kalamas, you yourselves know: 'These things are good; these things are not blameable; these things are praised by the wise; If you follow these teachings, they lead to benefit and happiness. So follow them and practice them.
AN 65 Gotama, the Historical Buddha
Doubt may be either intellectual and critical (kankhā) or a persistent negative scepticism or permanent indecisiveness − Sceptical Doubt (vicikicchā). Only this Sceptical Doubt is karmically unwise. It is such because it paralyses thinking and prevents progress. If we doubt everything, we won't take it onboard and so, cannot benefit from what is helpful. Reasoned, critical doubt on doubtful or unclear matters is intelligent behaviour. It is essential in Buddhism and in all subjects. However, a person with Sceptical doubts (vicikicchā) has doubts about Gotama (the Buddha), Buddhism, the other Buddhists, and the training. It is a kind of doubt that persists and does not respond to proof.
He or she doubts whether things are intelligent or not, necessary for progress or not. A person showing Sceptical Doubt is unwilling to think things out and come to a conclusion. He or she is uncertain, indecive and in two minds about the teachings.
Scepticism and Sceptical Doubt are not the same. Sceptics approach new knowledge with the intention to clearly understand what is claimed and to verify it. They approach new knowledge with a neutral viewpoint, which is changed to belief or disbelief, or remains neutral depending on further reasoning or experience. Sceptics can change their neutral state to faith based on reasoning and experience.
However, some faith is required in all change work,. You can judge the teaching and the teachers, but before you do something, you cannot know. You cannot know before you do. So some faith is required, whether you are learning Buddhism or any other subject.
In contrast with sceptics, people with Sceptical Doubt find it difficult to change their disbelief. They do not work to understand what is claimed, and are not affected by reasoning or experience. They always have further questions which are not satisfied by valid answers. It is almost as if they fear the truth. It seems they lack basic trust in others. There are different kinds of doubt, and one kind of doubt is common to almost any thoughtful person, whatever the matter being studied.
For instance, suppose someone obtained a treasure map. He carefully studies the map, the history, facts about the island where the pirates buried the treasure − he does everything he can to verify the facts. Everything fits together, but he cannot be absolutely sure it is a true map until he finds the treasure. A little doubt remains. Because he has faith (based on his research), he travels to the island, digs for the treasure and finds it. Previously, he had some small doubt and uncertainty. But now he he has none. The point of this story applies to Buddhism too. You are absolutely certain only when you have achieved it.
Everyone on the path has some degree of doubt. Doubt is finally removed at the higher levels, because then the Buddhist sees the truth. Like the man who has found the treasure, no doubt remains.