The definition of doubt is a mental factor that wavers with respect to its object.
When a primary mind is associated with doubt, the primary mind itself and all its other mental factors such as feeling, discrimination, and intention also take on the two-pointed quality of doubt. Doubts prevent us from engaging in either worldly or spiritual actions with confidence and resolve. There is a saying in Tibetan that just as we cannot sew with a two-pointed needle, so we cannot fulfil our wishes with a two-pointed mind. An essential prerequisite for success in our spiritual practices is unwavering faith and confidence. If we lack this we shall not be able to commit ourself wholeheartedly to a particular practice or maintain our effort long enough to accomplish any results. Therefore, it is essential to eliminate those doubts that interfere with the development of pure faith.
There are two types of doubt:
1 Deluded doubts
2 Non-deluded doubts
Both types of doubt are characterized by an indecisive wavering with respect to their object, but whereas deluded doubts disturb our mind, non-deluded doubts do not. Non-deluded doubts often occur when we are studying profound Dharma topics such as emptiness. In the process of trying to understand a difficult point we wonder ‘Does it mean this or does it mean that?’ This is doubt because it engages its object two-pointedly, but it does not disturb our peace of mind. On the contrary, this type of doubt can often arouse a strong interest in Dharma and lead to a thorough understanding of the subject. For example, it is said that just developing doubts about emptiness will damage samsara. Unfortunate beings do not even develop such doubts.
Deluded doubts, on the other hand, greatly disrupt our spiritual practice. They can shake our faith in Dharma, discourage us from study, and prevent us from succeeding in our meditations. Deluded doubts are the most common cause of abandoning Dharma practice. They also interfere with our worldly actions by rendering us incapable of making decisions, or of staying with a particular course of action. Since deluded doubts are so harmful it is important to recognize them as soon as they occur and to take steps to dispel them. (Geshe Kelsang Gyatso, Understanding the Mind: an Explanation of the Nature and Functions of the Mind, pp. 73-74, © 1993, 1997, 2002)