A fundamental part of the insight that allows us to make sense of experience is seeing how all that is known constitutes a unity.
There are several ways to make sense of the claim that all things form a unity. It is tempting to read this claim as a statement of material monism. If Heraclitus is a material monist, then the original material out of which everything else derives on his picture, is, doubtless fire. He speaks about fire a great deal, referring to it as the principle of wisdom and the material manifestation of the logos. Since the logos is only the physis in the sense of being the unifier in nature, perhaps fire, its material manifestation, is the physis in the sense of the material from which everything arose.
But though it is tempting to believe that Heraclitus was a material monist with fire as his physis, it seems unlikely. Fire seems like more of a metaphor for the logos (as we will see below) than its actual material manifestation. In addition, there are no fragments that directly link fire to the originating substance of the world.
A more likely interpretation of Heraclitus's claim that all things are one focuses on the paradoxes he presents rather than on his statements concerning fire. The paradoxes of opposites that Heraclitus presents fall into three broad sorts: First there are several paradoxes that seem intended to alert us to the co-existence of opposites. So, for instance, he points out that the same ocean water can be undrinkable and dangerous to us but drinkable and life sustaining to fish. The next bunch of paradoxes seems to point to a stronger relation between certain opposites, a metaphysical and conceptual dependence (i.e. they cannot exist, and cannot even be thought of, without one another). The road up and the road down, he points out, are the same road. One cannot exist without the other, nor can we think of one without thinking of the other. Similarly, night and day are dependent in this way. There would be no night without day and vice versa. Finally, in the last bunch of paradoxes, we get the strongest sort of relation, a relation of identity. God (or the logos), he tells us, is both night and day, winter and summer, war and peace, satiety and hunger.
It is this relation of identity between opposites that Heraclitus is probably referring to when he claims that all things are one. All things are one, because all opposites form a unity through their connection to the logos. Through all of the daily, seasonal, social, and other cycles, unity is maintained because everything is a part of the divine law of the logos. It is this fact—the unity of all things through the logos—that we have to understand if we are going to make sense of our experience. In the search for knowledge, in other words, the first step is to come to grips with the fact that what seems to be a clash of opposites is really just the unity of a rational pattern.
Like all of the Presocratics, Heraclitus is sure that there is an equilibrium in nature, some constant state that gets maintained, but unlike the others he believes that the equilibrium state is a state of constant flux. The cosmos is a place of constant change, with a hidden underlying stability in the form of the divine law according to which all change takes place. Returning now to the idea of fire, we can understand why Heraclitus identifies that substance as the most fitting metaphor for the logos: fire is a substance of constant change. It is for this same reason that Heraclitus likens the cosmos to a river; a river both constantly changes, as new water continually flows through, and remains the same (we continue to call the Rhine the "Rhine" from moment to moment).