When one cultivates strong concentration by means of tranquility or insight meditation, the mind is no longer distracted by thoughts and other hindra… - Mahasi Sayadaw

Uncertainty about what is wholesome and unwholesome is called skeptical doubt (vicikiccha). It is considered a hindrance to liberation as it hinders the arising of discriminating knowledge. A person who has doubts about the wholesomeness and unwholesomeness of phenomena is unable to abandon what is unwholesome, develop what is wholesome, and escape the cycle of suffering.

When intensively practicing insight, your first priority should be given to it, with the understanding that insight is the essential cause of liberation. ... You should not interrupt it for a minute or even a second.

Doubt about the fact that insight wholesomeness consists of simply observing the presently arising mental and physical phenomena is also skeptical doubt. This doubt is so subtle that it is rarely detected but is instead mistaken for investigation. This doubt masquerades as analytical knowledge. The commentary on Netti Yuttihāra explains as follows: "Doubt appears in the guise of investigation."