People have asked about my practice. How do I prepare my mind for meditation? There is nothing special. I just keep it where it always is. They ask, “Then are you an arahant?” Do I know? I am like a tree in a forest, full of leaves, blossoms and fruit. Birds come to eat and nest, and animals seek rest in its shade. Yet the tree does not know itself. It follows its own nature. It is as it is.
(Ajahn Chah, A Tree in a Forest)
So after that, Ven. Sona determined the right pitch for his persistence, attuned the pitch of the [five] faculties [to that], and there picked up his theme. Dwelling alone, secluded, heedful, ardent, & resolute, he in no long time reached & remained in the supreme goal of the holy life for which clansmen rightly go forth from home into homelessness, knowing & realizing it for himself in the here & now. He knew: "Birth is ended, the holy life fulfilled, the task done. There is nothing further for the sake of this world." And thus Ven. Sona became another one of the arahants.
"Is there, monks, any criterion whereby a monk, apart from faith, apart from persuasion, apart from inclination, apart from rational speculation, apart from delight in views and theories, could affirm the attainment of enlightenment: 'Birth is destroyed, the holy life has been accomplished, what was to be done is done, there is no further living in this world'?"
"For us, Lord, all things have the Blessed One as their root, their guide, their refuge. It would be well, Lord, if the meaning of these words were to be made clear by the Blessed One. Hearing it from the Blessed One, the monks will remember it."
"There is such a criterion, monks, whereby a monk... could affirm the attainment of enlightenment... What is that method?
"Seeing thus, the instructed disciple of the noble ones grows disenchanted with form, disenchanted with feeling, disenchanted with perception, disenchanted with fabrications, disenchanted with consciousness. Disenchanted, he becomes dispassionate. Through dispassion, he is fully released. With full release, there is the knowledge, 'Fully released.' He discerns that 'Birth is ended, the holy life fulfilled, the task done. There is nothing further for this world.'"
(According to the last quotation every Arahant know that he is an Arahant.)
Ajahn Chah said, that he doesn’t know whether he is an Arahant or not, but many suttas say that it’s impossible. So I think with this statement Ajahn Chah admitted that he is not an enlightened person.
Or, the other possibility: I don’t understand something crucial. (I must admit I feel so a bit.) Could you explain me why Ajahn Chah answered this way to that question?