All Experienced Events Are Conditional And Impermanent

It is for this reason that the Buddha declared that there is no "self." The word "self" is very slippery and is used in many ways, so to understand this famous Buddhist denial of self we first have to be clear what is being denied. The criterion of "self" is initially that it is supposed to be something constant, something that remains unchanged over time throughout all of our changing experience. We might say, "I was standing but now I am sitting; I was sad but now I am happy; I was a child but now I am an adult." Situations and experiences change, but this "I," something called "the self," is referred to in all of these statements as something that stays the same through all of these changes, somehow underlying them or connecting them. Sometimes people refer to their body as their self; when asked which person in a photo is "you," you might point to the picture of your own body to contrast it with the pictures of all the other bodies there: that one is me. The Buddha does not deny  that your body exists and is uniquely identifiable. He denies that this should count as "self," because self us supposed to be something that remains constant and unchanged over time. That identifiable body didn't exist before my birth and will not exist after my death, so it is clearly not an eternal self; but even during my present lifetime, on a micro level, there is no moment when it is not changing. It is impermanent, and thus it cannot be considered my self.

---Brook A. Ziporyn, in Emptiness and Omnipresence---