In general, a doubt arises because due to ignorance or the imprints of ignorance we do not know an object with a valid cognizer. Non-ascertaining perceivers, correct beliefs, and wrong awarenesses can all lead to doubts. For example, if we see a vase of flowers but cannot tell whether the flowers are real or artificial, this non-ascertaining perceiver may cause us to develop the doubt ‘Are these real flowers or artificial flowers?’ Until we realize an object in a completely reliable way, that is with a valid cognizer, there is always the possibility of doubts occurring.
The most common causes of deluded doubts are wrong views or the imprints of wrong views. Two beginners might listen to a teaching on karma, and one might believe it while the other might develop many deluded doubts about it. This indicates that the first person has imprints of believing in karma, or already believes some similar doctrine, while the second person has imprints from denying the law of karma in previous lives, or cherishes views that are incompatible with a belief in karma. Another reason why someone might doubt the teachings on karma is that he or she has no faith in the Teacher or in Buddha. If we have strong faith in the Teacher or in Buddha we shall believe the teachings, even if we do not fully understand them.
Yet another cause of developing doubts is incorrect investigation and analysis. For example, people with scientific training may find it difficult to believe in hidden objects because their existence cannot be proved by ordinary scientific methods. If they try to establish the existence of karma or enlightened beings, for example, by means of a scientific experiment they will be unable to do so, and as a result they may develop deluded doubts. The laws of karma and the existence of enlightened beings are hidden objects that are beyond the scope of scientific observation. If we want to investigate these phenomena to prove that they exist, we must use correct methods, such as the logical reasons that are presented in Dharma.
The causes of virtuous doubts are often the opposite of the causes of deluded doubts. While listening to teachings on profound subjects such as the twelve dependent-related links, for example, an intelligent and faithful student may develop more doubts than a dull student with less faith. The less capable student may be satisfied with a very rough understanding, but the intelligent one will want to understand it precisely and validly. He or she will analyze the subject carefully and ask many questions. Although we may never lose conviction in the truth of Buddha’s teachings, in the process of striving to understand them fully we may develop many doubts as to their exact meaning. If we have faith we shall yearn to penetrate the profound meaning of Dharma and not be content with a superficial understanding. Thus, faith encourages us to study deeply and ask many questions. Skilful questioning based on an appreciation of the profundity of Buddha’s teachings can increase faith, whereas unskilful questioning based on scepticism can destroy it. (Geshe Kelsang Gyatso, Understanding the Mind: an Explanation of the Nature and Functions of the Mind, pp. 75-77, © 1993, 1997, 2002)