Which was the first philosopher to think of ET?

Question

Looking at the sky, we seen the moon, stars, sun and planets. Their motion have been plotted since Early Babylonia, and gods and godesses have been associated with them.

When, so we are told, men began to think about Astronomy rather than Astrology, and Materiality rather then Mythology in Hellenic Antiquity, did they then begin to suppose that these moving objects in the sphere of heaven may also contain life? If they did what reason did they put forward for that? Or did it happen much later, say in the late Renaissance, after Galileos discovery of the Moons of Jupiter. It certainly must have happened before Jules Verne novel of 1865 - de la Terre a la Lune (from the Earth to the Moon).

(Returning to the scene of mythology - did one then think, say that goddess of the moon, Selene lived on the moon? Or the god of the Sun - Helios lived on the Sun? Probably not - I suppose they were seen as embodiments, or the spirit of the Sun or Moon).

This isn't really metaphysics, as is usually understood, but I couldn't think of anymore appropriate tags. I suppose I see the question of life like us elsewhere, that is not on this planet being a fundamental question of natural philosophy (physikoi) as understood in these terms in Early Philosophy. After all, once it was deduced that the earth was a sphere, as was done in Hellenic Antiquity, and presumably hanging in space; and seeing that the Sun & Moon are also spheres - the natural question would have been are they like Earth. One might assume that the Sun was discounted as being too hot. But that does leave the moon as a possibility.

Answer 1

The debate goes back to Greek Antiquity. Cosmic Pluralism

was a corollary to notions of infinity and the purported multitude of life-bearing worlds were more akin to parallel universes (either contemporaneously in space or infinitely recurring in time) than to different solar systems. After Thales and his student Anaximander opened the door to an infinite universe, a strong pluralist stance was adopted by the atomists, notably Leucippus, Democritus, and Epicurus.

but

their opponents—Plato and Aristotle—had greater effect. They argued that the Earth is unique and that there can be no other systems of worlds. This stance neatly dovetailed with later Christian ideas and pluralism was effectively suppressed for approximately a millennium [in the West]

in the Islamic world:

Fakhr ad-Din ar-Razi (1149–1209), [the Persian Theologian & Philosopher] in dealing with his conception of physics and the physical world in his Matalib, rejects the Aristotelian and Avicennian notion of the Earth's centrality within the universe, but instead argues that there are "a thousand thousand worlds (alfa alfi 'awalim) beyond this world such that each one of those worlds be bigger and more massive than this world as well as having the like of what this world has." To support his theological argument, he cites the Qur'anic verse, "All praise belongs to God, Lord of the Worlds," emphasizing the term "Worlds."

and this was portrayed in a tale of the Arabian One Thousand & One Nights, The Adventures of Bulukiya, which depicted a cosmos consisting of different worlds, some larger than Earth and each with their own inhabitants.

It was only with the beginnings og the Copernican revolution in the West that Cosmic Pluralism asserted itself again in the West, with early contribution made by the Domincan friar & Philosopher, Giordano Bruno who affirmed:

an infinite God necessarily created an infinite universe, formed of an infinite number of solar systems, separated by vast regions full of aether, because empty space could not exist.

It was Brunos theology, rather than his cosmology had him burned at the stake for heresy. The Catholic church had no official position on the then new Copernican cosmology.

During the late 16th century, and throughout the 17th century, Bruno's ideas were held up for ridicule, debate, or inspiration. Margaret Cavendish, the English aristocrat & philosopher for example, wrote an entire series of poems against "atoms" and "infinite worlds" in Poems and Fancies in 1664.

But his cosmology was popularised later by de Fontanelle, in Entretiens sur la Pluralite des Mondes - on the Plurality of Worlds, at which point Cosmic Pluralism entered the mainstream of European thought.

Answer 2

Perhaps I am missing the point of the question but what we call mythology was believed to be the 'truth' back in the day. Whether Valhalla or Olympus, the Hindi Ramayana or the African Dogon visitor from Sirius; ancient thoughts teemed with what many today would call extraterrestrial. As philosophers and shaman are essentially indistinguishable from each other, I would posit alien life forms have been an interest of mankind at the moment a civilization recognised the difference between planets and stars.