Considering his body in light of the four elements which were thought to make up all matter, the Buddha said,
(In conversation with my teacher, he said he found it odd that the Buddha assigned feces to the element of water, not earth. I think the assignment says a great deal about the diet of the medicants in that day, and its effects on the digestive system.)
“This body of mine is a combination of the four elements. Its hair, nails, teeth, skin, flesh, muscles, bones, and marrow belong to the earth. Its saliva, tears, pus, blood, snot, froth, phlegm, semen, urine, and feces belong to water. Its warmth belongs to fire. And its movement and stillness belong to wind. Take away each of the four elements, and this body turns out to be an illusion. Where is it now?” (Complete Enlightenment Sutra)
We now know, of course, that the building blocks of all matter are much more complex than the four elements, but this new knowledge only reinforces the Buddha's point. If I'm made up of organs, which are composed of tissues, which are composed of cells, which are composed of organic chemicals, which are made up of atoms, which consist of neutrons, protons and electrons, which are made up of quarks and neutrinos and other sub-atomic particles, which are composed of, I don't know, strings they tell me - then on what level does life exist? On what level does the self reside? If I'm alive, are my bodily chemicals alive? Are their atoms alive? It they're not, then how can I, made up of non-living elements, be myself a living being?
If it's so hard to find life, so much the harder it is to find the ego, the personality, the self.
In the Majjhima Nikaya, or Middle-Length Discourses, the Buddha said:
"If there really existed an Ego, there would be also something which belonged to the Ego. As, however, in truth and reality, neither an Ego nor anything belonging to an Ego can be found, is it not really an utter fool's doctrine to say: This is the world, this am I; after death I shall be permanent, persisting and eternal?"The Buddha here discounts an Ego-self existing in this world, as well as a soul or self existing in the afterlife. These teachings resonate throughout Buddhism and Zen, to the point where teachers often claim that the Ego-self is just an illusion, that belief in the existence of the Ego-self is a delusion. You, it is claimed, don't exist!
So when Mahakasyapa, out in the sun mixing mud, said, "If I don't do this, who else will do it for me?" it was a complete refutation of this philosophical idea of no-self, an acceptance of the obvious in the very concrete world of, well, mixing concrete.