In the long sojourn of philosophy on the earth there has existed hardly a philosopher or historian of philosophy who did not mention Thales and try to characterize him in some way. He is generally recognized as having brought something new to human thought. Mathematics, astronomy and medicine already existed. Thales added something to these different collections of knowledge to produce a universality, which, as far as writing tells us, was not in tradition before, but resulted in a new field, science.
Ever since, interested persons have been asking what that new something is. Answers fall into (at least) two categories, the theory and the method. Once an answer has been arrived at, the next logical step is to ask how Thales compares to other philosophers, which leads to his classification (rightly or wrongly).
The most natural epithets of Thales are "materialist" and "naturalist", which are based on ousia and physis. The Catholic Encyclopedia goes so far as to call him a physiologist, a person who studied physis, despite the fact that we already have physiologists. On the other hand, he would have qualified as an early physicist, as did Aristotle. They studied corpora, "bodies", the medieval descendants of substances.
Most agree that Thales' stamp on thought is the unity of substance, hence Bertrand Russell:
"The view that all matter is one is quite a reputable scientific hypothesis."
"...But it is still a handsome feat to have discovered that a substance remains the same in different states of aggregation."
Russell was only reflecting an established tradition; for example, Nietzsche, in his Philosophy in the Tragic Age of the Greeks, wrote:
"Greek philosophy seems to begin with an absurd notion, with the proposition that water is the primal origin and the womb of all things. Is it really necessary for us to take serious notice of this proposition? It is, and for three reasons. First, because it tells us something about the primal origin of all things; second, because it does so in language devoid of image or fable, and finally, because contained in it, if only embryonically, is the thought, 'all things are one.'"
This sort of materialism, however, should not be confused with deterministic materialism. Thales was only trying to explain the unity observed in the free play of the qualities. The arrival of uncertainty in the modern world made possible a return to Thales; for example, John Elof Boodin writes ("God and Creation"):
"We cannot read the universe from the past..."
Boodin defines an "emergent" materialism, in which the objects of sense emerge uncertainly from the substrate. Thales is the innovator of this sort of materialism.
Thales represents something new in method as well. Edmund Husserl attempts to capture it as follows. Philosophical man is a new cultural configuration based on a rejection of tradition in favor of an inquiry into what is true in itself; that is, an ideal of truth. It begins with isolated individuals such as Thales, but they are supported and cooperated with as time goes on. Finally the ideal transforms the norms of society, leaping across national borders.
The term, Pre-Socratic, derives ultimately from Aristotle, a qualified philosopher ("the father of philosophy"), who distinguished the early philosophers as concerning themselves with substance. This is not entirely true.
Diogenes Laertius on the other hand took a strictly geographic and ethnic approach. Philosophers were either Ionian or Italian. He used Ionian in a broader sense, including also the Athenian academics, who were not Pre-Socratics. From a philosophic point of view, any grouping at all would have been just as effective. There is no basis for an Ionian or Italian unity. Some scholars, however, concede to Diogenes' scheme as far as referring to an "Ionian" school. There was no such school in any sense.
The most popular approach refers to a Milesian school, which is more justifiable socially and philosophically. They sought for the substance of phenomena and may have studied with each other. Some ancient writers qualify them as Milesioi, "of Miletus."
Thales had a profound influence on other Greek thinkers and therefore on Western history. Some believe Anaximander was a pupil of Thales. Early sources report that one of Anaximander's more famous pupils, Pythagoras, visited Thales as a young man, and that Thales advised him to travel to Egypt to further his philosophical and mathematical studies.
Many philosophers followed Thales' lead in searching for explanations in nature rather than in the supernatural; others returned to supernatural explanations, but couched them in the language of philosophy rather than of myth or of religion.
When you specifically look at the influence Thales had in the pre-Socrates era, he was one of the first thinkers who thought more in the way of logos than mythos. The difference between these two more profound ways of seeing the world is that mythos is concentrated around the stories of holy origin, while logos is concentrated around the argumentation. When the mythical man wants to explain the world the way he sees it, he explains it based on gods and powers. Mythical thought does not differentiate between things and persons and furthermore it does not differentiate between nature and culture. The way a logos thinker would present a world view is radically different from the way of the mythical thinker. In its concrete form, logos is a way of thinking not only about individualism, but also the abstract. Furthermore, it focuses on sensible and continuous argumentation. This lays the foundation of philosophy and its way of explaining the world in terms of abstract argumentation, and not in the way of gods and mythical stories.
Our sources on the Milesian philosophers (Thales, Anaximander, and Anaximenes) were either roughly contemporaneous (such as Herodotus) or lived within a few hundred years of his passing. Moreover, they were writing from a tradition that was well-known. Compared to most persons, places and things of classical antiquity, we know a great deal about Thales. Most modern dissension comes from trying to interpret what we know.
Diogenes Laertius lists two works written by Thales, and also relates the strange tradition that he did not write. Diogenes, however, had access to two of Thales' letters, which he quotes. Those writings are two more than the surviving works of Socrates, which are none. And yet, thanks to Plato, we know as much about Socrates as anyone. More than likely, the non-writing tradition about Thales is a complaint that such a famous man did not leave enough to be quoted by the secondary sources.
The main secondary source concerning the details of Thales' life and career is Diogenes Laertius (DL here), "Lives of Eminent Philosophers". This is primarily a biographical work, as the name indicates. Compared to Aristotle, DL is not much of a philosopher. He is the one who, in the Prologue to that work, is responsible for the division of the early philosophers into "Ionian" and "Italian", but he places the Academics in the Ionian school and otherwise evidences considerable disarray and contradiction, especially in the long section on forerunners of the "Ionian School." DL does give us the extant primary sources on Thales (the two letters and some verses).
Most philosophic analyses of the philosophy of Thales come from Aristotle, an Academic and a professional philosopher, tutor of Alexander the Great. Aristotle may or may not have had access to the now mysterious possible works of Thales. There was also an extensive oral tradition. Both the oral and the written were commonly read or known by all educated men in the region.
Academic philosophy had a distinct stamp: it professed the theory of matter and form, which modern scholastics have dubbed hylomorphism. Though once very widespread, it was not generally adopted by rationalist and modern science, as it mainly is useful in metaphysical analyses, but does not lend itself to the detail that is of interest to modern science. It is not clear that the theory of matter and form existed as early as Thales, and if it did, whether Thales espoused it.
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