One of the very known suttas in Pali Canon is SN 35.23. Unfortunately very often the sutta is misunderstood. Too many Buddhists tend to think that the sutta means nothing exists beyond six spheres of perception. This is not so.
An excellent comment of Thanissaro Bhikkhu:
The Commentary’s treatment of this discourse is very peculiar. To begin with, it delineates three other “All’s” in addition to the one defined here, one of them supposedly larger in scope than the one defined here: the Allness of the Buddha’s omniscience (literally, All-knowingness). This, despite the fact that the discourse says that the description of such an all lies beyond the range of explanation.
Secondly, the Commentary includes nibbana (unbinding) within the scope of the All described here — as a dhamma, or object of the intellect — even though there are many other discourses in the Canon specifically stating that nibbana lies beyond the range of the six senses and their objects. Sn 5.6, for instance, indicates that a person who has attained nibbana has gone beyond all phenomena (sabbe dhamma), and therefore cannot be described. MN 49 discusses a “consciousness without feature” (viññanam anidassanam) that does not partake of the “Allness of the All.” Furthermore, the following discourse (SN 35.24) says that the “All” is to be abandoned. At no point does the Canon say that nibbana is to be abandoned. Nibbana follows on cessation (nirodha), which is to be realized. Once nibbana is realized, there are no further tasks to be done.
Thus it seems more this discourse’s discussion of “All” is meant to limit the use of the word “all” throughout the Buddha’s teachings to the six sense spheres and their objects. As the following discourse shows, this would also include the consciousness, contact, and feelings connected with the sense spheres and their objects. Nibbana would lie outside of the word, “all.” This would fit in with another point made several times in the Canon: that dispassion is the highest of all dhammas (Iti 90), while the arahant has gone beyond even dispassion (Sn 4.6; Sn 4.10).
This raises the question, if the word “all” does not include nibbana, does that mean that one may infer from the statement, “all phenomena are not-self” that nibbana is self? The answer is no. As AN 4.174 states, to even ask if there is anything remaining or not remaining (or both, or neither) after the cessation of the six sense spheres is to differentiate what is by nature undifferentiated (or to objectify the unobjectified — see the Introduction to MN 18). The range of differentiation goes only as far as the “All.” Perceptions of self or not-self, which would count as differentiation, would not apply beyond the “All.” When the cessation of the “All” is experienced, all differentiation is allayed