the world was fire, which was subject to change – that makes him a materialist monist. From fire all things originate and all things return to it again in a process of eternal cycles. Fire becomes water and earth and vice versa. These everlasting modifications explain his view that the cosmos was and is and will be. The idea of continual flux is also met in the "river fragments". There, Heraclitus claims we can not step into the same river twice, a position summarized with the slogan ta panta rhei (everything flows). One fragment reads: "Into the same rivers we both step and do not step; we both are and are not" (DK 22 B49a). Heraclitus is seemingly suggesting that not only the river is constantly changing, but we do as well, even hinting at existential questions about humankind. Another key concept of Heraclitus is that opposites somehow mirror each other, a doctrine called unity of opposites. Two fragments relating to this concept state, "As the same thing in us is living and dead, waking and sleeping, young and old. For these things having changed around are those, and those in turn having changed around are these" (B88) and "Cold things warm up, the hot cools off, wet becomes dry, dry becomes wet" (B126). Heraclitus' doctrine on the unity of opposites suggests that unity of the world and its various parts is kept through the tension produced by the opposites. Furthermore, each polar substance contains its opposite, in a continual circular exchange and motion that results in the stability of the cosmos. Another of Heraclitus' famous axioms highlights this doctrine (B53): "War is father of all and king of all; and some he manifested as gods, some as men; some he made slaves, some free", where war means the creative tension that brings things into existence. A fundamental idea in Heraclitus is logos, an ancient Greek word with a variety of meanings; Heraclitus might have used a different meaning of the word with each usage in his book. Logos seems like a universal law that unites the cosmos, according to a fragment: "Listening not to me but to the logos, it is wise to agree (homologein) that all things are one" (DK 22 B50). While logos is everywhere, very few people are familiar with it. B 19 reads: [hoi polloi] "...do not know how to listen [to Logos] or how