We read in the Kindred Sayings (IV, Salayatana-vagga, Third Fifty, chapter V, 152, Is there a method?) that the Buddha said:
Is there, monks, any method, by following which a monk, apart from belief, apart from inclination, apart from hearsay, apart from argument as to method, apart from reflection on reasons, apart from delight in speculation, could affirm insight thus: Ended is birth, lived is the righteous life, done is the task, for life in these conditions there is no hereafter?
For us, lord, things have their root in the Exalted One Well indeed were it if the meaning of this that has been spoken were to manifest itself in the Exalted One. Hearing it from him the monks will remember it.
There is indeed a method, monks, by following which a monk could affirm insight And what is that method?
Herein, monks, a monk, seeing an object with the eye, either recognizes within him the existence of lust, aversion and ignorance, thus: I have lust, aversion and ignorance, or recognizes the non-existence of these qualities within him, thus: I have not lust, aversion and ignorance. Now as to that recognition of their existence or non-existence within him, are these conditions, I ask, to be understood by belief, or by inclination, or hearsay, or argument as to method, or reflection on reasons, or delight in speculation? Surely not, lord.
Are these states to be understood by seeing them with the eye of wisdom?
Then, monks, this is the method by following which, apart from belief a monk could affirm insight thus: Ended is birth for life in these conditions there is no hereafter.
The same is said with regard to the doors of the ear, the nose, the tongue, the body and the mind.