Heraclitus makes the statement that "you can't step into the same river twice." This comes from the idea that the river is flowing and ever changing. With its constant flow, by the time you put your foot in take it out and put it back in again, the water you first touched will already be traveling down the river. For Heraclitus, reality is just like this changing flowing river. In reality, things are always changing and time flies by with the blink of an eye. Time does not stop, it is a constant flow of life and once it is gone we cannot get that moment back.
No matter what we do, things in life are constantly changing. Change occurs because the elements are fighting against one another to be the one. These changes are occurring because the one substance we are trying to get, is working against itself in a way. The four elements of fire, water, earth, and air are in a constant war mutilating the purity of the one, being the solitary structure of reality itself. Though these elements are in opposition of each other, they are united in some way shape or form. When we examine Heraclitus' example of not being able to step into the same river twice, we can notice that a river is one thing. That river will always remain that one thing, a river, even though it is constantly flowing and changing. This is the basis of Heraclitus' statement, the fact that reality, and the river, has ceaseless change.
As noted before, all substances are fighting against each other, yet at the same time they are united. Heraclitus believes that there is one single aspect of reality that is the cause of the short-lived stability of things and that is the uniting source of all things, fire. Fire is the source of all things. It is a substance that is ever changing, it flickers and transforms before our very eyes. It builds itself up, yet it can completely deteriorate things down to complete nothingness. Heraclitus views fire as "something" that is parallel to "nothing.