Heraclitus believed things were ever-changing, and that may be true. Science and physics( which is an arm of philosophy tells us that when force is applied to things there is the possibility of a change in the molecular make up of the item. It is like a formless matter. Once the matter has been molded into a particular form it is more than likely to lose atoms during the process. I believe the example of the flowing river is a pretty clever one. Being that the river is ever flowing there is constant erosion occurring as the constant (the bed of the river) interacts with the moving (the flow of the water). In actuality even the small acts such as shaking hands involves the exchange of atoms and molecules.
Parmenides presented a conflicting philosophical opinion to that of Heraclitus. Parmenides presented the view that the state of being in nature is constant. It does not change and that our perception of reality may at times be very deceitful. While I do not agree with this in regards to the state of being and nature I do think this argument would hold much weight and would be considered a solid truth in terms of psychology. A persons psychological makeup could very well affect the way a person views reality, and could present falsehoods. One of Parmenides most popular argument of that something that is not cannot be feasibly proven as it is not in a state of being. I would argue that it could simply as the inverse of something that is.
While both have left a longing impression on the western philosophy and we are still arguing the same debate that they did today, I would have to agree with the argument of Heraclitus on the topic of the status of being. Things are always changing; we live with gravity which in itself causes us to change, without it we would not age nearly as quickly as we do. I find the difference in the argument in the duality and constant being of nature to be one of a matter from a modern perspective as looking at things from a macro and micro perspective.
On the macro side things look the same and unchanged as it takes drastic force or influence to change things, but on the micro level even the small of acts cause for a strong movement of atoms. I would have to agree with Heraclitus, although Parmenides does present a very valid argument when placed in proper context.