The Nyayasutras assert that perception is the primary proper means of gaining true knowledge. All other epistemic methods are directly or indirectly based on perception, according to the text, and anything that is claimed to be "true knowledge" must be confirmed or confirmable by perception. This it terms as the doctrine of convergence, and this doctrine includes direct or implied perception. Gautama defines perception as the knowledge that arises by the contact of one or more senses with an object or phenomenon. Gautama dedicates many sutras to discuss both the object and subject in the process of perception, and when senses may be unreliable. Erratic eyesight or other senses ("Avyabhicara") can be a source of doubt or false knowledge, as can prejudgmental or prejudicial state of mind, states the Nyayasutras.