Heraclitus said this to some young men who came to see him in his remote hermitage. He had acquired a reputation for being irascible and distant, really because nobody understood his ideas which were far ahead of their time. A theory of constant change: A form of monism based on energy, and he often used as an example the energy of fire. Of necessity he spoke in elliptic fashion using allegory, for example “You cannot step into the same river twice, for all is constant change!” In a sense it was not until Spinoza that there was another philosopher expressing these ideas!
He only ever wrote his ideas down once, on a scroll deposited in the library in Ephesus which is now lost. So we know of his doctrines from quotations and references in the writings of others.
These young men were charmed by him however as he patiently explained his philosophy the long day through; and they remembered that day for the rest of their lives. What they experienced was far from the ideas they had of him when they approached his abode. The only way people could rationalize Heraclitus’ independent thinking was that he had some kind of miraculous contact with the gods that were the basis of the Greek religion of the time. His greeting was aimed at putting these fears to rest and welcoming them as fellow-enquirers and equals. People rarely visited him so fearsome was his reputation! He had a child-like quality about him and had often played with the children in the town square. He had great sympathy with the freshness of spirit of the children and their openness to new ideas.
In a real sense the approach used by Heraclitus is the same as that used by modern physics to explain how our universe works. The link between matter and energy and the development of the concept of spacetime having been postulated by Einstein at the beginning of the last century. The developments in our understanding continue to the present day. Most recently with the confirmation of the existence of the Higgs boson. Modern science uses the same openness and patient enquiry based oh the evidence to hand that Heraclitus used.