What is the legal ruling regarding doubt about everything, and what is its remedy?

Whoever experiences frequent doubts should not surrender to the whisperings and temptations of devils. If the doubt occurs during prayer and is not a recurring experience, he should complete his prayer and perform two prostrations of forgetfulness (sujood as-sahw) after the salutation. If the doubt occurs after finishing the prayer, he should disregard it. However, for one who is constantly afflicted by doubt, he should not act upon what he doubts and should perform two prostrations of forgetfulness before the salutation. Among the means of treatment are: maintaining congregational prayers and occupying one's time with beneficial knowledge or righteous deeds.