Readers of my books have occasionally written to me in the hope I can prove to them that the doctrines there presented are true. This I regret being unable to do. I can prepare a powerful case, and in some instances have tried to do so; but, like all cases, a trained mentality should be able to demolish them. This is as it must be, but it should be remembered that the powers of the logical intellect are--and must always remain--limited by the amount of factual material and experience available to it. There is only one way anyone can be certain of the truth of these statements. He must take to the Quest and keep on the Quest, until the Overself reveals itself to him. Indeed, more people know in their own personal histories the concrete proof of these statements than the public generally dreams of.
-- Notebooks Category 12: Reflections > Chapter 2: Philosophy and Contemporary Culture > # 104