The logos, according to Heraclitus, is that universal principle which governs all things. In fragment B1, he declares: "As for this logos which is ever true, men fail to understand it, either before hearing it or once they have heard it. For, although all things happen in accordance with this logos, they resemble inexperienced people when they experience words and actions such as I set forth, distinguishing each thing according to its nature and stating how it is. But other men do not know what they do when awake, just as they forget what they do when asleep." Then he adds in fragment B2: "Therefore one must follow what is common. But, although the logos is common, most men live as if they had a private understanding." Fragment B50 sums up the essence of his thought: "It is not by listening to me, but to the logos, that it is wise to agree that all things are one." Finally, fragment B114 establishes a link between the logos and divine law: "Those who speak with understanding must rely on what is common to all, like a city on its law, and even more strongly. For all human laws are nourished by one divine law, for it rules as far as it wills, and suffices for all, and prevails."

Th doctrine of universal flux is expressed in several fragments. Fragment B12 states: "Into the same rivers, ever-different waters flow upon them." Fragment B49a specifies: "Into the same rivers we step and do not step, we are and we are not." And fragment B91 adds: "It is not possible to step twice into the same river, for it scatters and gathers, it approaches and recedes."

The struggle of opposites is at the heart of cosmic harmony. Fragment B8 sets forth: "That which opposes unites, and the finest attunement arises from opposites, and all things come about through strife." Fragment B53 proclaims: "War is the father of all things, the king of all things; some it has shown to be gods, others men; some slaves, others free." Fragment B80 insists: "One must know that war is common, that justice is strife, and that all things come to pass through strife and necessity."

Fire, the fundamental principle, appears in fragment B30: "This world, the same for all, no god nor man has made, but it ever was and is and will be: an ever-living fire, kindling in measures and going out in measures." Fragment B64 declares: "The thunderbolt steers all things." And fragment B66 announces: "Fire, when it comes upon them, will judge and consume all things."

The soul is also a recurring theme. Fragment B36 explains: "For souls, death is to become water, and for water, death is to become earth, but from earth water is born, and from water the soul." Fragment B45 reveals: "You would not find out the boundaries of the soul, even by traveling along every path, so deep a logos does it have." Fragment B115 adds: "To the soul belongs a logos that increases itself."

Wisdom and the divine are evoked in fragment B32: "The One, the only wise, is willing and not willing to be called by the name of Zeus." Fragment B41 specifies: "Wisdom is one thing: to know the thought by which all things are steered through all." And fragment B102 affirms: "To God, all things are beautiful, good, and just, but men have deemed some things unjust and others just.

Finally, sleep and death are addressed in fragment B26: "A man in the night lights a light for himself, when his sight is extinguished; living, he touches the dead in his sleep, and awake, he touches the sleeper." Fragment B21 concludes: "Death is what we see awake, and what we see asleep is sleep.