Buddhism's Dynamic Theory of 'Self'.

Master Dogen famously said that 'to study the buddha-way is to study the self. To study the self is to forget the self. To forget the self is to be actualised by every single thing'.

So what constitutes 'self' is very important in his teachings, as it is in other perspectives and philosophies.

The Buddhist theory of self is called paṭicca-samuppāda in Pali, which is generally translated as 'dependent origination'.

Basically, it presents a twelve-fold chain of stages whereby things come into being. The theory itself is quite detailed, but the Buddha summed it up like this:

When there is this, that is. 

With the arising of this, that arises. 

When this is not, neither is that. 

With the cessation of this, that ceases.

Basically, dependent origination points to the fact that our sense of self arises (or doesn't!) with every single thing in our experience - what we are perceiving here and now, and what we are thinking and feeling, is us, so it is a very direct and dynamic view of an ever-changing 'self'.

In terms of our direct experience of this - we might have a memory, a mental image from the past, that evokes feelings in the body that we label 'fear', 'anger', 'regret', 'happiness' or whatever, and we interpret that process as being 'me' due to a basic underlying tendency to construct a self. Likewise we might perceive objects in our experience and assign them values such as 'good' or 'bad' in relation to this sense of self.

In zazen however we stop this activity, letting all things that make up our experience just come and go, 'forgetting the self', and thereby we see things as they actually are - free of any underlying self, and they affirm our mutual nature as open and free of any restricting 'self', 'cessation' as it is called in the translations of the ancient Buddhist texts.

In that way, all things 'inner' and 'outer'... thoughts, feelings, solid objects, sounds... can be experienced to directly express the same selfless freedom, without the fearful clinging that underlies much of the mental and emotional activity by which we mispercieve things as being, or relating to, a sense of self.

Here's a good article on dependent origination:

https://www.buddhistinquiry.org/article/dependent-origination/