The bulk of criticisms levelled at mysticism have to do with the fact that modern mystics have been forced into claiming that theirs is a different kind of knowledge or a different way of knowing, and that neither is subject to empirical verification in the same way that ordinary knowledge about our world is. They have fallen into this trap as a result of having had no clear, logical, penetrating understanding of their subject. They have been forced into this intellectual cul-de-sac because there exist no recent, coherent theories to account for the fundamental religious experience. It is sincerely hoped that, with the advent of this understanding, mysticism will no longer be such a mystery.
We experience Nirvana by employing ordinary, every-day awareness - the very same awareness we use to experience ordinary, every-day things, including happiness. In fact, we are always permanently and inescapably aware of our moods whatever they may be; but the untrained mind's potential for experiencing high degrees of happiness is frustrated by excessive conscious mental activity - particularly of the random, habitual variety. On the other hand, in the mind of the expert meditator (during his best sessions of meditation), the prevailing mood is one of perfect happiness, undiluted by any conscious mental activity. The fact that Nirvana is an extreme instance of the mood of happiness creates difficulties when we come to reconcile it with normal manifestations of the mood but it remains true to say that Nirvana is an extrapolation (to perfection) of ordinary happiness, even though difficult to recognise as such because of its intensity. I pity the person who has to admit that he has never been happy. Nirvana is different from that experience only in its intensity. It is not a different kind of knowledge nor is it a new way of knowing.
However we are still faced with the task of empirically verifying it. But we should have no more difficulty verifying it in its extreme form (if only as a logical possibility) than we have in verifying that humans can experience any degree of happiness. I maintain that a general consensus exists which forces us to conclude that we are sometimes happy and we are sometimes sad. Who would deny that moods exist? If a child tells us s/he (we need a neutral pronoun) is sad and backs up his claim by crying bitterly, who could deny s/he is telling the truth? More importantly, we have each felt it for ourselves often enough. There can be no doubt that happiness exists, therefore perfect happiness exists.
Speaking more generally, it is true to say that not all data comes to us via our sensory apparatus. Moods and emotions come to us from within our brains while pains and pleasures come to us from within our bodies. These categories of experience do not rely on our sensory apparatus for their detection but are never-the-less just as real as empirically verifiable data originating in our environment which can be verified by our senses and corroborated by others in the same manner.
G E Moore and A J Ayer were two philosophers who had a good deal to say about "good" but could not logically ground it. However both introduced concepts which were designed to recommend good acts to us in preference to bad ones. Moore said ethical statements had "intrinsic value" while Ayer said they had "emotive force". They intuitively recognised that "good" acts are characterised by an amplification of the physical condition of happiness in our brains but failed to recognise the authenticity (as real data) of moods they themselves experienced when good was done. Had they made this simple connection they would have been easily able to logically ground "good". Actions are deemed to be good because, ultimately, fewer neurons fire - and we feel a whole lot better for it.