This sutta gives many valuable lessons on practical issues surrounding the attempt to develop an internal meditative dwelling of emptiness, to maintain it, and to see it through to Awakening. Some of these issues include the need for seclusion as a conducive setting for the practice, types of conversation and thinking that are beneficial and harmful for the practice, the dangers of being distracted by visitors, and the proper attitude to have toward one's teacher. However, for an explanation of emptiness in and of itself, it's necessary to look elsewhere in the Canon.

There you find emptiness approached from three perspectives, treating it (1) as a meditative dwelling, (2) as an attribute of objects, and (3) as a type of awareness-release. The first approach is obviously the most immediately relevant to the discussion in this sutta, but in fact all three approaches play a role here.

Emptiness as a meditative dwelling is most fully discussed in MN 121. Essentially, it boils down to the ability to center the mind in a particular mode of perception, to maintain it there, and then to notice the absence and presence of disturbance within that mode. The process starts with perceptions of one's external surroundings — village, wilderness, the earth property — and then moves internally to the four formless states, the "themeless concentration of awareness," and finally to release from all mental fermentation. Each step is compared to the one preceding it to see how its more refined perception engenders less disturbance. For instance, if you move from a perception of the wilderness to a perception of earth, the first step is to settle and "indulge" in that perception. Then you notice what types of disturbance have been abandoned in the move from the perception of wilderness to the perception of earth — for example, all thought of the dangers of wilderness are gone — and then to see what disturbances remain based on the latter perception. Then you abandon the perception causing those disturbances and move on to a more refined level of perception. This process is pursued until it arrives at the "themeless concentration of awareness." When noting that even this refined level of concentration is fabricated, inconstant, and subject to cessation, one gains total release from all mental fermentations and the disturbances that would arise based on them. This is the level of emptiness that is "superior and unsurpassed," and is apparently what the Buddha is referring to in this sutta when he says that by "not attending to any themes, he enters & remains in internal emptiness."

Notice that in every step along the way of this process, the emptiness is the lack of disturbance experienced in a particular mind state. This means that the mind state is to be perceived simply as an example of the presence and absence of stress. In other words, emptiness in this sense relates directly to the second of the three characteristics — stress or suffering. The pursuit of this emptiness relates to the four noble truths, as it looks for the causes of stress and uses tranquility together with insight to abandon those causes in a quest to put a total end to suffering.

Emptiness in its second meaning, as an attribute of objects, is most fully discussed in SN 35.85. That sutta describes emptiness as meaning the lack of self or anything pertaining to a self in the internal and external sense media. Whatever sense of self that may surround these objects is not inherent in them, and is instead simply the result of one's own penchant for "I-making" and "my-making." Seeing the artificiality of "I-making" and "my-making" in this way helps lead to a sense of disenchantment with these "makings," thus helping to abandon any clinging associated with them.

Thus emptiness in this sense relates directly to the third of the three characteristics: not-self. However, just as the three characteristics are not radically separate from one another — everything stressful is for that reason not-self — the practical application of this sense of emptiness is not radically different from the first. As SN 12.15 points out, when one no longer latches onto any idea of "my self," one sees phenomena within and without simply as examples of stress arising and passing away. To practice meditation from this perspective — seeing each state of concentration as an example of stress arising and passing away — is to develop emptiness as a meditative dwelling.

Emptiness in its third meaning, as a type of awareness-release, is an application of emptiness in its second. MN 43 describes this state of concentration as follows: "There is the case where a monk — having gone into the wilderness, to the root of a tree, or into an empty dwelling — considers this: 'This is empty of self or of anything pertaining to self.'" It adds that this awareness-release is different from the awareness-release that results when one doesn't attend to any themes. Thus this state of concentration cannot be entirely equated with the emptiness as a meditative dwelling mentioned in this sutta. MN 106 further adds that if one frequently abides in the emptiness awareness-release, one may either attain the dimension of nothingness — one of the formless states — or be committed to the discernment that will lead to Awakening. The first of these two alternatives is another way in which emptiness as an awareness-release differs from emptiness as a meditative dwelling as defined in MN 121. However, because the standard definition of discernment is seeing phenomena in terms of the four noble truths, the second alternative — being committed to discernment — would apparently follow the same pattern suggested by SN 12.15, above. In other words, as one no longer perceives phenomena in terms of self, one tends to view them simply as examples of stress arising and passing away. So, again, this third meaning of emptiness, like the second, eventually leads in practice back to the first. As MN 43 notes, when one attains full awakening, the themeless awareness-release and the emptiness awareness-release come to differ only in name, and not in actuality.

In reading the following sutta, you will notice that the various meanings of emptiness will fit some contexts better than others. Still, it is important to remember that in the course of practice, all three meanings are related and all will inevitably play a role in Awakening.