Doubt brings about lasting understanding; doubt is not an end in itself. What is true is revealed only through doubt, through questioning-the many illusions, traditional values, ideals.
– Verbatim Reports of Talks and Answers…Adyar, India 1933-34 p.29
If you doubt, that is, if you desire greatly to find out, you must let go of those things which you hold so dearly. There cannot be true understanding by keeping what you have. You cannot say, “I shall hold on to this prejudice, to this belief, to this ceremony, and at the same time I shall examine what you say.” How can you? Such an attitude is not one of doubt; it is not one of intelligent criticism.
– Verbatim Reports of Talks and Answers…Adyar, India 1933-34 p.30
The Buddha advises seekers of truth not to accept anything merely on the authority of another but to exercise their own reasoning and judge for themselves whether a thing is right or wrong.
On one occasion the citizens of Kesaputta, known as the Kalamas. approached the Buddha and said that many ascetics and brahmins who came to preach to them used to exalt their own doctrines and denounce those of others, and that they were at a loss to understand which of those worthies were right.
“Yes, O Kalamas, it is right for you to doubt, it is right for you to waver. In a doubtful matter, wavering has arisen,” remarked the Buddha and gave them the following advice.
“Come, O Kalamas, do not accept anything on mere hearsay (i.e. thinking that thus have I heard it from a long time). Do not accept anything by mere tradition (i.e., thinking that it has thus been handed down through many generations). Do not accept anything on account of rumours (i.e., by believing what others say without any investigation). Do not accept anything just because it accords with your scriptures. Do not accept anything by mere supposition. Do not accept anything by mere inference. Do not accept anything by merely considering the appearances. Do not accept anything merely because it agrees with your preconceived notions. Do not accept anything merely because it seems acceptable (i.e., should be accepted). Do not accept anything thinking that the ascetic is respected by us (and therefore thinking it is right to accept his word).
– Anguttara Nikaya I gradual sayings, Kalama Sutta (The bracketed explanatory parts of the translation are in accordance with the commentary and sub commentary)