The Heart Sutra is not a general statement about existence, it is a statement about the nature of our existence as Buddhist practitioners. It is not a philosophical treatise, it is a description of Zazen. That is why we chant it after we sit.
The most famous part of the Heart Sutra is “form is emptiness and emptiness is form.” Someone who doesn’t know about the Heart Sutra might think that ‘form’ means the physical elements of the material world, things. But that is not what is meant. The Heart Sutra is named such because it is an abbreviation of the teachings on emptiness. ‘Form’ in this context is the first of the five Skandhas. The five Skandhas are a description of you. The form isn’t the form of things generally, it is your form.
That form – the word for form is ‘rupa’ – also appears as the fourth link (as namarupa – name and form) in the chain of dependent origination in early Buddhism. The idea is that the whole causal network of suffering is dependent upon a sequence of causes, each of which is dependent on the others. Break one and you break the chain of suffering. What form means in that context is our tendency to create durable, knowable entities out of the flow of experience. Once we do that, identifying, naming and reifying them, that creates desire, clinging and so on, other links of the chain.
The ‘form is emptiness’ statement is an abbreviation. If we unfold the Sutra, it would also be telling us that the other Skandhas are equally empty: sensation is empty, perception is empty, mental formations are empty and consciousness is empty. Adding these four makes it easier for us to understand what might be meant by ‘emptiness’. If we pay proper attention to our sensations for example, we see that each sensation, far from being something fixed, is energetic, fluctuating, changeable and relational. It is related to other sensations, to perception and consciousness and so on.
So it is easy for us to understand how the four Skandhas other than form are empty. But an understanding of that does not necessarily get us out of the primary dualism of self and world, which is the true origin of our suffering. It might well make us more embodied, and it might make us more integrated, but it doesn’t necessarily deal with that split. But regarding our form – my body- as empty (relational, interdependent) does directly attack that dualism, which is why it is particularly important, especially now, when there is a risk of meditation being swallowed up by the highly individualistic – and trivial – ideal of self improvement.
It seems to me that in our ordinary life, when we regard this body, it is as if we are seeing it in a mirror or, in some other way, picturing it. In Zazen we are not picturing it, at least not all the time. In breaking the mirror of the self, Zazen enables us to experience this body as interdependent, fluctuating, not separate. That is the foundation for us to start to see in a different way. Through an attention to this form, this body, the form of the world, likewise, can change.