Xenophanes was a phusikoi in the Milesian tradition, but he is not counted among the Milesians because he was not actually from Miletus. He was born instead in Colophon, not far from Miletus, around the year 570 B.C. After Colophon fell to Medes, Xenophanes left the city, becoming a wandering poet and philosopher. It is not known exactly where he traveled, but it seems likely that he visited southern Italy at some point, since he is clearly familiar with the Pythagorean doctrine of the transmigration of souls. In addition, some historians claim that he was the teacher of Parmenides, who was himself a native of southern Italy.
Xenophanes' interests were varied. He apparently wrote on purely poetic matters, even authoring a work on how to prepare for a drinking party (symposium), but he was also very interested in natural philosophy and religion. He was adamant in rejecting the Olympian account of the gods, insisting instead that there is only one, non-anthropomorphic god who is unmoving, but all-seeing, all-hearing, and all thinking, and who controls the universe with his thought. It seems plausible that his theological views were in a sense similar to the theological views of the Milesian philosophers, who all seemed to attribute some sort of divinity to their physis. Xenophanes is primarily of note because he was the first philosopher to explore the epistemological (i.e. having to do with knowledge) implications of the new philosophical mode of investigating the world.
Unlike the Milesian material monists, Xenophanes posited two physis. Unfortunately, not much evidence regarding his natural science remains. It is entirely unclear why he chose these two elements as his physis.
Most of the evidence we have regarding Xenophanes' thought involves his attack on the traditional view of the gods. He is especially concerned to prove that the traditional conception of divinity is the result of the human tendency to project our own nature onto the gods. Each of the different races, he points out, believes that the gods look like them (e.g. the Egyptians claim the gods are flat-nosed and drak, the Thracians claim the gods are blue-eyed redheads). In addition, Homer and Hesiod attributed all sorts of human personality flaws to divinity. If animals could draw, Xenophanes jibes, then horses would draw gods that look like horses, and oxen would draw gods that look like oxen.
Two fragments show Xenophanes explaining away supposed divine beings in natural terms. The being that the Greeks call Iris, the messenger goddess, is nothing but a cloud, and the weird lights that can be seen from sea, which have been traditionally been explained as twin gods, are also clouds.
Xenophanes' attack can be seen as an expression of the philosophical break with the poetic, mythological tradition. The philosophers are not atheists, but they do not believe in anthropomorphic gods who concern themselves with human affairs and society. What, then, do they believe in? Xenophanes, at least, believes in one god who controls the world with his thought. This god does not move physically (though probably he is within the physical world, perhaps in the form of the two physis) but all of him sees, thinks, and hears. This God might be something like rationality within nature, much like Hercaclitus's logos or even Aristotle's final cause or teleological principle.
In overturning divine authority and poetic license for rational thought, the Milesians turned single-mindedly to the observation of evidence. They were unreflective empiricists, gathering all their knowledge through experience, collecting the data to be explained. Xenophanes follows in this empiricist tradition, but he is reflective about it. In particular, he notices that it has some dire consequences for the possibility of human knowledge.
When knowledge came from divine authority, the limit of knowledge was just the limit of what the gods wanted to reveal, or the limit of what the imagination could drum up. The philosophers have hit now on a new, improved method of obtaining knowledge: investigating the world for themselves. The only problem, Xenophanes purports to show, is that this method does not actually yield much knowledge; the best it can yield is true belief. This is because most subjects of investigation—the gods, the physis, the derivation of plurality from unity—cannot be observed. These matters go beyond our experience. If the only way to obtain knowledge is to gather data with the senses (which Xenophanes believes it is) then we cannot obtain knowledge about the most important things, theology and science.
In addition, Xenophanes points out, we can even disagree about what is directly perceived. As Xenophanes says, "if god had not created honey we'd say figs are much sweeter" (fragment 21B38). In other words, there is a high degree of indeterminacy to our perceptions, a subjective element in all of our observations. We do not gain access to the true nature of, say, the fig by tasting it. Rather, our perception of the taste of the fig varies with our other experiences. If we have tasted honey, then the fig does not taste so sweet; if we have not tasted honey, then the fig tastes very sweet to us. There is, in other words, a veil of appearances or perceptions that we cannot go beyond in our experience; all that we have access to is our own perceptions and these are subjective: they do not accurately reflect the objective reality of things.
Since we rely on experience to give us knowledge, and experience lets us down in these two ways (first, by not even extending to the most important subjects, and second by denying us access to the real, objective nature of things) we are doomed to be forever without any real knowledge. Xenophanes' final analysis of the human capacity for knowledge is as skeptical as it could be.