The Milesians and Xenophanes all assumed that there was only candidate available to take the place of divine authority and poetic inspiration: straight empirical observation. As we just saw, the reliance on straight empirical investigation lead Xenophanes to conclude that real human knowledge was impossible in most fields of inquiry; if all one has to go on is observation, then where direct observation is unavailable, knowledge is impossible. Probably in response to this pessimistic conclusion, Heraclitus was the first thinker to propose an alternative to straight observation. He presents an epistemological theory of tempered empiricism, arguably much like the method of inquiry we use in science and philosophy today. According to this picture, observation is still important in the search for knowledge, but reason allows the observer to go beyond the observational given. This epistemological theory is tightly connected to Heraclitus's interesting metaphysics, according to which the world is ordered, guided, and unified by a rational structure, a single divine law, which he calls the "logos". Matching the divine cosmic logos, happily, is a logos that resides in each of our souls. Our private logos (presumably something like our faculty of reason) allows us access to the divine logos, and thus reopens the possibility of human knowledge. Observation without an understanding of the logos is useless, but observation coupled with an understanding of the logos yields true knowledge.
Heraclitus was born in about 540 B.C. to one of the aristocratic families of Ephesus, near Colophon. His noble birth brought with it an important hereditary role in the life of the city, a position that involved responsibilities as both a political and religious leader (for instance, he would have been in charge of supervising the city's official sacrifices). Heraclitus, however, had no interest in the political life, nor in traditional religion, and he handed over his hereditary ruling position to his younger brother. Throughout his life, and well after his death, Heraclitus had a reputation as a misanthrope and as a deliberately obscure thinker. His reputation as a misanthrope was probably based reasonably on the unkind words he had for other philosophers and historians (he called everyone from Homer to Xenophanes an ignoramus, which they technically were according to this theory of knowledge). His reputation as an obscure thinker, on the other hand, is probably unjustified. Though his lists of paradoxes might seem obscure on their surface, it is only because they are only intelligible when seen in their connection to the logos.
The basic tenet of Heraclitus's system is the claim that there is a rational structure to the cosmos and that this rational structure orders and controls the universe. The logos is Heraclitus's physis but only in the sense of a unifier in nature: a fundamental part of understanding the logos involves seeing that all things are unified in it. The logos, however, is presumably not the material out of which everything else arose, though it is the origin of all things insofar as it is the arrangement of all matter.
Heraclitus often refers to the logos as the mind of God, though it is not clear what implications this has for his theory. Probably, Heraclitus simply identified the logos with the mind of God because it is the controlling, rational force within nature. Certainly he does not view the logos in any sort of anthropomorphic terms, and it is an entirely natural, rather than supernatural, force. In addition, the logos exists squarely within the physical world. Oddly, Heraclitus seems to view the logos as part of the world in the same sense that water or air is a part of the world. It is as if he is treating the recipe as one of the ingredients.
The logos is not only the basic concept of Heraclitus's metaphysics, it is also the basic principle of his epistemology. It is only though understanding the logos that we can make sense of our experience. Though the logos is an independently existing truth available to all (a fact which he underscores by speaking about a logos in each human soul), most people fail to recognize it. In two highly vivid metaphors, Heraclitus describes the folly of those multitudes who attempt to investigate nature without understanding the logos. He compares these people first to sleepers; like sleeping minds, the mind that does not understand the logos cannot receive information from the outside world. What goes on in a sleeping mind is purely subjective and is not connected to what is going on in the real world. Similarly, those who investigate nature without understanding the logos only gain access to their own subjective worlds, not to the real, objective one. (It is interesting to view this metaphor in relation to Xenophanes' claim about the inherent subjectivity of sensory perception).
Later, Heraclitus also compares these same people to barbarians—that is, to people who do not understand the Greek language. When you do not understand a language, Heraclitus is telling us, all that you hear in the words is noise; you cannot discern the underlying order, the meaning of these words. In our experience of the world we are confronted with something like a language, and most people fail to make sense of this language because they do not understand the logos (which is the language of nature). Therefore, to these people (among whom he counts all previous philosophers and poets) observation is nothing but meaningless noise. We cannot, in other words, simply gather facts as the Milesians and Xenophanes tried to do, but rather in order to obtain any knowledge from these facts we must understand how they relate to the logos.