Buddhadasa’s book on Anapansati, Mindfulness of Breathing, is a complete manual on meditation on the breath, from the beginning stages, with methods for developing initial concentration on the breath, to development of more intense levels of concentration, and then methods for shifting into the four Jhana states. But he notes early on that one does not need to attain high levels of concentration, or attempt the Jhana states, in order to do Vipassana. A foundational level of concentration is enough.

When the conditions (paccaya) determining the physical and non-physical—and otherwise Mind-and Matter (nama-rupa)—are scrutinized with such fineness as to penetrate clearly into the mode and conditions of their arising, the breathing becomes even more subtle. And when the impermanence, unsatisfactoriness and non-selfhood of Mind-and Matter are discerned, the breathing becomes even more subtle and calm. . .(p. 136) Then to attain the climax of his [concentration] practice, he should carry on to practice the last Tetrad (Stages) XIII to XVI, which deals with the penetration of impermanence, unsatisfactoriness, and non-selfhood. On the other hand, if the meditator does not wish to reach the higher stages of tranquility meditation (First to Fourth Absorptions [jhanas]), preferring to develop insight immediately and directly, he may do so. What he must do in that case is to change the concentration on breathing into scrutinizing of Mind-and-Matter in terms of impermanence and so on as mentioned above/ Then by the power of intuitive scrutiny alone, he may proceed directly to the last Tetrad—all of which will be discussed in its proper place. . . (p. 137) So anyone who wishes to develop insight directly does not need the absorptions. He just needs a limited amount of concentration to serve as a basis for the insight, because he his aiming at the cessation of suffering and does not care for any special ability or quality, such as Higher Supernormal Powers (abhiñña) or the like. This technique of scrutinizing in order to develop insight will be explained in detail later (Stages XIII, XIV). (p. 138).

In the last chapter on the Fourth Tetrad, he presents a method for Vipassana, and explains that it consists in a contemplation of Impermanence, while maintaining concentration on the breath. Impermanence implies also non-self, unsatisfactoriness and sunyata or ’emptiness’.

“Generally speaking, to contemplate impermanence means to contemplate and see [experience] the arising, continuation and ceasing of all things. . . . (p. 541). And the contemplation of impermanence, while maintaining concentration on the breath, is vipassana.

One contemplates the Five Aggregates (as objective experience) and the Six Sense Bases (as subjective experience), in the order of Dependent Origination, all as arising and passing, all as impermanent. One does this not as an intellectual analysis, but by watching the contents of one’s mind as these ‘external’ and ‘internal’ mental objects arise and decay and cease during meditation.

Further, in the next stage of contemplation, one sees impermanence in a more subtle way: one sees that all the different things in the world, whether material or mental, whether within the body or without, all equally depend on one single instant of consciousness (or mind, citta, that is, a mind which is performing the function of contacting or sensing an object through the eye, ear, etc.). Different things are known in the world because there is consciousness sensing them. If no consciousness arises, then the different things in the world are in effect non-existent. [I would say ‘non-object’.] On this ground, the arising of mind (i.e. arising of feeling or sensing them) means therefore the arising of all objects (or their being felt or sensed). As soon as mind (citta) ceases, they too cease, being non-existent for the individual concerned. We can therefore say that everything depends on mind, is in the power of the mind, or has significance only because of consciousness or mind, and is always arising and ceasing with the arising and ceasing of consciousness. Thus since consciousness is something which is arising and ceasing every moment, it follows that all things, whether material or mental, inside or outside the body, are arising and ceasing every thought moment too. (p. 538).

I would add that consciousness or mind is also impermanent. being both one of the Five Aggregates and one of the Six Sense bases that is contemplated as impermanent. The arising of Mind-consciousness is dependent on contact with the world. Mind-consciousness and world arise together, in mutual causality. It’s a two-way street: when world ceases, mind-consciousness also ceases.

When the world is described as ‘like a dream’, like the reflection of the moon in a lake; like the image in a mirror’: it’s not the material reality of the world that changes or becomes immaterial; it’s that your perception of the material world changes. Instead of things looking solid, they start to look translucent, ‘like a dream’, like the reflection of the moon in a lake, etc.

It’s a shift in perspective from the particle to the wave. The particles are still particles, i.e. solid material reality, but they act like waves, and you see them as waves. Waves are still waves, but they act like particles and you sense them as particles. It is one reality, perceived in different ways. It’s not just a momentary shift, but a gradually sustained shift from looking at the self-world as objects to looking at them as waves. One begins to sense one’s own being ‘smeared’ across reality, just as an electron is ‘smeared’ across a field of probabilities as it orbits around the nucleus. Self-world becomes a probability wave.