There are no surviving texts of the Pre-Socratics, and very little is known about their lives. What is known comes to us from the writings of other philosophers, beginning with Plato (c. 428–c. 348 B.C.E.) and Aristotle (384–322 B.C.E.), their contemporaries, and especially Aristotle’s student Theophrastus (371–c. 287 B.C.E.). For example, the writings of Heraclitus (535–475 B.C.E.) consist of “fragments,” and there are only 450 enduring lines from Empedocles (c. 490–430 B.C.E.). Because we have no primary sources, we can’t be certain how much of what is related about the Pre-Socratics is skewed by the biases of their interpreters.