[...] Na literatura grega mais antiga que sobreviveu, e na tradição poética até ao século IV a.C., aither era o termo básico para aquilo a que hoje chamamos ar. Aer, por outro lado, era originalmente apenas um exemplo muito isolado de ar: névoa ou nuvem obscura. Gradualmente, o seu significado foi-se alargando, passando de névoa ou vapor a descrever a atmosfera vaporosa que respiramos, e assim a designar o ar ou a atmosfera em geral. À medida que o âmbito do termo aer aumentava, o âmbito da palavra aither diminuía; no início do século IV a.C. era apenas utilizado para designar a região mais elevada e exaltada do ar, nos céus. Por outras palavras, em vez de aer ser um exemplo particular de aither, aither tornou-se um exemplo particular de aer. Foi então que, depois de a palavra se ter dissociado no espírito das pessoas do ar em geral, aconteceu o inevitável: iniciou-se uma fase de especulação autoconsciente sobre a substância em que consistia o aither. Em termos de pura influência na opinião geral na Antiguidade posterior, o resultado mais importante desta especulação foi a opinião dos estoicos de que o ar é uma forma de fogo.

Não podia deixar de acontecer que os gregos posteriores, sem aquilo a que chamamos discernimento crítico, lessem o seu próprio entendimento de aither para os poetas e filósofos anteriores; temos de ter cuidado para não repetir o mesmo erro. Os danos foram mais ou menos reparados no caso dos poetas, mas o mesmo não se pode dizer dos filósofos. Isto deve-se, em parte, ao fato de se misturarem coisas que, embora intimamente relacionadas, não são a mesma coisa. É evidente que existe uma relação estreita entre a região dos céus e o calor do sol (que, na Antiguidade, se supunha ser feito de fogo); no entanto, quando, na cosmologia primitiva, a relação entre o sol e o ar foi revelada, tratava-se de uma correlação e não de uma identidade. O atraso no esclarecimento da questão dos filósofos resulta também da forma como a filosofia grega passou a ser tratada academicamente como uma disciplina separada da literatura grega, e como estando de alguma forma sujeita a princípios e regras diferentes. O desejo de ver a filosofia ocidental como uma tradição contínua que caminha para uma sofisticação, uma autoconsciência e uma compreensão cada vez maiores implica inevitavelmente olhar para os primeiros filósofos gregos através dos olhos dos posteriores. Aristóteles e os estoicos continuam a determinar a perspetiva básica, apesar de ser do conhecimento geral a sua total falta de história. O resultado é o caos. Por exemplo, continua a assumir-se que Parménides "implicou" a equação aither — fogo ao descrever o seu princípio do fogo como "aitherial". De fato, como mostra o contexto, o objetivo do adjetivo é definir o fogo como 'brilhante' em contraste com a escuridão da noite, e também definir o fogo como um princípio 'celestial' — pertencente às alturas em contraste com o princípio da noite que é pesado e pertence a baixo". Isto não justifica de forma alguma a conclusão de que, para Parménides, ambos consistem em fogo. Pelo contrário, quando ele usa o termo aither, a sua referência a ele — juntamente com a terra, o sol e a lua — como "comum" é impossível de explicar satisfatoriamente na suposição de que aither é fogo. No entanto, faz sentido imediato e óbvio quando, como no caso de todos os outros escritores dos séculos VI e V, aither é aceite como referindo-se às "regiões superiores e inferiores do ar... evidentemente pensadas como um continuum que se estende desde a superfície da terra até às estrelas ou mais além".

In later antiquity it was generally taken for granted that when Empedocles spoke of aither he meant the element of fire. Recently it has been common to assume instead that for Empedocles aither was not an element at all, but a secondary combination of fire plus air. Both these assumptions are tributaries of an even larger and more enduring one. This is that Empedocles’ basic name for the fourth element—in addition to fire, water, and earth—was aer: the origin of our word ‘air’.

On the one hand, this major assumption made it inevitable that sooner or later Empedocles’ Hera would be equated with air. The equation of Hera with aer may well already have been suggested before Empedocles’ time; it crops up a century after him in Plato’s Cratylus as one possible explanation for Hera’s name, and it was later to have a flurry of success among the Stoics. Greeks—like any other ancient people—loved playing with words, and with the passing of time a philosophical significance was bound to become attached to the fact that, as spelt in Greek, Hera is an anagram of aer? On the other hand, equation of Empedocles’ air with Hades was equally inevitable. There was an obvious precedent for identifying them in the Homeric description of Hades’ realm as ‘air-like darkness’ (ζόφος ἠερόεις): an expression which originally meant ‘misty darkness’ but eventually—as aer came to mean air instead of mist—would be explained as meaning ‘dark realm of air’.

On either interpretation the basic assumption is that Empedocles called his fourth element aer. Ironically, the last person who seems to have been consulted on the matter is Empedocles himself. In what survives of his work he explicitly lists the four elements several times. Once he refers to the fourth element—apart from fire, water, and earth—as ‘heaven’ (ouranos). But every other time the fourth element is not aer] it is aither. The classic example is the statement that ‘we see earth with earth, water with water, aither with aither, and fire with fire’. So much for the claim—made in antiquity, still repeated today—that aer was Empedocles’ name for the element of air; and so much for the claims that for Empedocles aither was either the element of fire or a secondary combination of fire plus air.

How did it come about that Empedocles has been so radically misunderstood? The answer is simple. In the earliest surviving Greek literature, and in poetic tradition down to the fourth century BC, aither was the basic term for what we now call air. Aer, on the other hand, was originally only a very isolated example of air: obscure mist or cloud. Gradually its meaning broadened from mist or vapour to describing the vaporous atmosphere we breathe, and so to denoting air or atmosphere in general. As the scope of the term aer increased, the scope of the word aither decreased; by the early fourth century BC it was only used to refer to the highest and most exalted region of air, up in the heavens. In other words instead of aer being a particular example of aither, aither had become a particular example of aer. It was then, once the word had become dissociated in people’s minds from air in general, that the inevitable happened: a phase of self-conscious speculation began about what substance aither consisted of. In terms of sheer influence on general opinion in later antiquity, the most important result of this speculation was the view of the Stoics that aither is a form of fire.

It was bound to happen that later Greeks, lacking what we call critical insight, would read their own understanding of aither back into the earlier poets and philosophers; we must be careful not to repeat the same mistake. The damage has more or less been repaired in the case of the poets; but the same cannot be said for the philosophers. This is partly a result of lumping together things that, even though intimately related, are not the same. There is an obviously close connection between the region of the heavens and the heat of the sun (which was generally assumed in antiquity to be made of fire); however, when in early cosmology the relationship between sun and aither was brought out into the open it was one of correlation, not identity. The delay in getting the record straight where the philosophers are concerned is also a result of the way that Greek philosophy has come to be treated academically as a separate discipline from Greek literature, and as somehow subject to different principles and rules. Desire to view western philosophy as a continuous tradition moving towards ever greater sophistication, self-consciousness, and understanding inevitably involves looking at early Greek philosophers through the eyes of later ones. Aristotle and the Stoics still determine the basic perspective, even though it is common knowledge how totally unhistorical they could be. The result is chaos. For example, it continues to be assumed that Parmenides ‘implied’ the aither — fire equation in describing his principle of fire as ‘aitherial’. In fact, as the context shows, the point of the adjective is to define the fire as ‘bright’ in contrast to the darkness of night, and also to define fire as a ‘celestial’ principle—belonging on high in contrast to the principle of night which is weighty and belongs down below.” This in no way justifies the conclusion that for Parmenides aither consists of fire. On the contrary, when he does use the term aither his reference to it—alongside earth, sun, and moon—as ‘common’ is impossible to explain satisfactorily on the assumption that aither is fire. However it makes immediate and obvious sense when, as in the case of all other sixth- and fifth-century writers, aither is accepted as referring to the ‘upper and lower regions of the air... evidently thought of as a continuum extending from the earth’s surface to the stars or beyond’.