A more lucid illustration of the fact that the descent into suññatā is gradual and collateral with experience, can be found in the Cūla-suññata Sutta (MN 121). There we see Ānanda inquiring of the Buddha with some diffidence whether the Lord actually meant what he said, when he declared at the township of Nagaraka that he used to spend most of the time ‘abiding in the void’ (suññatāvihārena). The Buddha reassures Ānanda, adding that not only in the past, but at present also, he is used to abiding in the void. Then he proceeds to explain the ‘true, unperverted and pure descent into the void’ (yathābhuccā avipallatthā parisuddhā suññatāvakkanti). In keeping with his empirical approach, the Buddha begins from his immediate surroundings: the Monastery of Pubbārāma itself.
“As this palace of Migāra’s mother is empty of elephants, cows, horses and mares, empty of gold and silver, empty of assemblages of men and women, and there is only this that is not emptiness, that is to say, the oneness grounded on the order of monks, even so Ānanda, a monk, not attending to the perception of village, not attending to the perception of human beings, attends to the oneness grounded on the perception of forest. His mind is satisfied with, pleased with, set on and freed in, the perception of forest. He comprehends thus: ‘The disturbances that might be resulting from the perception of village do not exist here; the disturbances that might be resulting from the perception of human beings do not exist here. There is only this degree of disturbance, that is to say, the oneness grounded on the perception of forest.’ He regards that which is not there as empty of it. But in regard to what remains there, he comprehends, ‘This is’ because it is. Thus, Ānanda, this comes to be for him a true, unperverted and pure descent into emptiness…” — Cūla-suññata Sutta (MN 121)
In much the same manner, the Buddha describes how a monk gradually and by stages attains to the perception of the earth as the object of meditative absorption (pañhavisaññā), the perception of the infinity of space (ākāsānañcāyatanasaññā), the perception of the infinity of consciousness (viññāṇañcāyatanasaññā), the perception of nothingness (ākiñcaññāyatanasaññā), the perception of neither perception nor-non-perception (nevasaññānāsaññāyatanasaññā), and the mental concentration based on the signless (animittaṃ cetosamādhi). At the last mentioned stage, he knows that he is experiencing only those forms of disturbances (darathā) arising from the body endowed with the six sense-spheres, due to the fact that he is living. Then again he reflects on the mental concentration on the signless, and his mind delights and abides therein.
He now begins to reflect: “This concentration of mind that is signless, is effected and thought out. But whatever is effected and thought out, that is impermanent and liable to cease.” Even as he knows and sees thus, his mind is released from the cankers of sense-pleasures, of becoming and of ignorance. In freedom, he has the knowledge that he is freed and he comprehends that he has attained the Goal. He introspects and finds that while those disturbances that might arise from the three cankers are no longer there, he is still subject to whatever disturbances that might arise from his body with its six sense-spheres due to the fact that he is alive. Accordingly he determines the fact of voidness, being faithful to the findings of his introspection. The Buddha sums up the discourse by asserting that this is the true, unperverted, pure and supreme descent into voidness:
“Ānanda, whatever contemplatives and brahmans who in the past entered and remained in an emptiness that was pure, superior, and unsurpassed, they all entered and remained in this very same emptiness that is pure, superior, and unsurpassed. Whatever contemplatives and brahmans who in the future will enter and remain in an emptiness that will be pure, superior, and unsurpassed, they all will enter and remain in this very same emptiness that is pure, superior, and unsurpassed. Whatever contemplatives and brahmans who at present enter and remain in an emptiness that is pure, superior, and unsurpassed, they all enter and remain in this very same emptiness that is pure, superior, and unsurpassed.
“Therefore, Ananda, you should train yourselves: ‘We will enter and remain in the emptiness that is pure, superior, and unsurpassed’.”