The earliest appearance of speculation on extraterrestrial life appears to belong to ancient Greece. The presocratic philosophers Democritus and Leucippus proposed an infinite universe made up of tiny, indivisible atoms (atomos means "unsplittable" in Greek). If the atoms could arrange in a way to produce life here on Earth, in an infinite universe, it seemed inevitable that they could arrange themselves to produce life elsewhere. These ideas, and also the idea that the universe is free and uncontrolled (an atheistic view) came to be known as Atomism.
Aristotle suggested the opposite, that the Earth is unique, and that there is only one cosmos. He believed that outside of the cosmos was the final cause, his diety. This diestic view led Aristotle to believe that there is a final cause for everything that occurs in the universe. Aristotle argued that everything in the universe, all matter, tended towards a middle point. Due to his own observations of what would later become known as gravity, he concluded that objects had a tendency towards the middle of the Earth, which implied under his belief that the Earth must be the middle of the universe. Earth remained at the center (geocentrisim), and because there can be only one center, there can be only one earth. He also believed that the cosmos had natural motion rather than violant motion, and that different types of matter were made out of different particles, as opposed to all objects being made out of the same kind of particles. This overall theory became known as Aristotelianism. Aristotle therefore denied plurality, and his view dominated Western thought for centuries, although proponents of the atomic worldview (for example Lucretius) surfaced from time to time.
During the medieval period, the Aristotelian view predominated, although scholars such as Aquinas speculated about the existence of extraterrestrial life. Nicole Oresme made the most interesting suggestions. There could have been successive single universes over time, or perhaps there were nested Earths, each with a common center. Furthermore, the latter suggestion would not violate Aristotle's theory of physics. Nicole Oresme also argued the idea that size is relative. He stated that any body is considered small in comparison to a larger body and vice versa, and that a larger body doesn't have more parts than a smaller body because all bodies have an infinite number of parts. With this theory, he stated that the world and everything within it could increase in size to be a thousand times larger or smaller, and we wouldn't know the difference because everything would still be proportional to its original size.
Nicholas of Cusa rejected Aristotle's beliefs that the earth and the heavens are fundamentally different. He developed the idea of relative motion and believed that earth was equivalent to other celestial bodies. The homogeneity of the universe would thus result in pluralism and support the belief that other lifeforms exist.
Plurality: The old word used to refer to the idea of extraterrestrial life. Proponents of plurality supported extraterrestrial life, opponents rejected it.
Unity of World: There is only one world.
Presentism: Putting present day values, ideas, and beliefs into interpretations of the past. Most Historians try to avoid presentism because people of the past had different beliefs and values than people today.
Contexualism: The opposite of presentism. Contexualism is putting old values, ideas, and beliefs in into the context of the time period that they originated in and followed. Contexualism lets historians maintain an unbiased opinion of past concepts.
Principle of Plenitude: The idea that the universe is essentially full, that there is an infinite amount of matter and space, such that life outside of the Earth seems inevitable.
Ontology: The study of existence/of being/of what exists. For example: saying that atoms exist is making an ontological claim
Epistemology: The study of knowledge and its foundations (why you know what you know and what it means to know that). Epistem in greek means knowledge.
Three basic types:
1. Empricism- the most secure form of knowledge is through experience (ex. science)
2. Rationalism- thinking or thought is the most secure form of knowledge (ex. math)
3. Revelation- knowledge is revealed to someone through divine interaction (most important to religion)
Empricism and rationalism are not mutally exclusive people/disiplines just tend to lean towards one or the other.
Cosmology: The study of the universe as a whole.
Cosmogony: Any theory which deals with the creation of the universe. (The common belief today is the Big Bang Theory.)
The Atomism of Democritus: An Introductory Essay by Dr. Jan Garrett of Western Kentucky University