According in itself (lecture 2 slides). Thales believes

According to Thales, he believes water is the substance that is the Arch? of all things. Arch? is a Greek word translated to ‘origin’ but in philosophy, Aristotle established the word Arch? as a word meaning the element or the principle of a thing that provides the conditions of the possibility of that thing which is intangible in itself (lecture 2 slides). Thales believes water is Arch? because it is the most basic substance that everything derives from and decays back to. He is interested in defining Arch? because of the cosmogony theory which is the theory of the origin of the world. Since Thales is interested in developing a theory of the origins of the world, he thought water would be best suited since he thought it is the stuff from which everything in the natural world originated. The main part of Thales’ theory is that he approaches Arch? with a naturalistic approach rather than a supernatural account. We, as humans, are made up of water so everything else in the world must be as well and because of this, water must be what everything in the natural world is originated from. Overall, Thales ultimately decided that since nourishment of things wet or just hot itself lives off of water, it must be that “water is the principle of the nature of moist things” (Thales 11A12). This is what he builds his theory of Arch? off of but does not realize the potential gaps in this theory.  

            Anaximander, on the other hand, deviates from Thales’ theory that water is Arch?, but his theory is also single and simple. He explains the orderly nature and Arch? of the natural world without external, supernatural principles just as Thales did but Anaximander thinks that Arch? is not water or any other element, it is Aperion (Hippolytus 12A11). Anaximander attempts to fill the gaps in Thales’ theory concerning change of Arch? as water with the concept of Aperion as a non-observable solution. Aperion is “an infinite and qualitatively indefinite substance that is neutral between the ‘elements’ (water, air, fire, earth) – it is neutral between opposed qualities associated with these elements, e.g. hot/cold or wet/dry” (lecture 2 slides). The gap in Thales’ theory is that it does not account for change. It does not explain how things come to be or perish or things having opposing properties. Anaximander deconstructs this puzzle by saying that there must be opposing qualities in the natural world for things to exist. If water (which is naturally cold) is Arch? then dry, warm, or non-moist things cannot exist, but they do so water is not Arch?. This might be solving the philosophical problem with Thales’ account because with Aperion it is not observable, Arch? is some nature from the heavens that is eternal and ageless (12A11).  

            Anaximenes’ account of Arch? is that he believes it to be air. His theory of Arch? as air is qualitatively indefinite and is a real natural and observable thing which helps us to make sense of Arch? more. Anaximenes Arch? that is air is seemingly infinite as well as it is qualitatively neutral. He tries to take into account both Thales’ and Anaximander’s views on Arch? by air being a more observable element such as water but holds similar qualities to Aperion like being eternal. The form of air is “when it is most even, it is invisible, but it is revealed by the cold and the how and the wet, and by its motion. It is always moving, for all the things that undergo change would not change if it were not moving” (13A7).