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Gravity is of course something that we can all observe. Stuff falls towards the ground. But not everything: some things like steam or smoke defy this force and instead float up.
During Ancient Greek and Roman times (when the two shared a lot of culture and scientific knowledge), how was this natural force explained? What was believed to be the cause of gravity? And why was it that some things obeyed it while steam and smoke defied it?
See Aristotle's Natural Philosophy.
According to Aristotle, change in the natural world can be :
[either] in accordance with the nature of the object — in which case the change is natural (phusei) or according to nature, or can happen in the face of a contrary disposition on the part of the nature of the entity — in which case the change is forced or contrary to nature.
A mover can effect a motion which is contrary to its own nature. Aristotle’s example of such an unnatural mover is the lever, an object heavy by nature, with which loads can be lifted.
[The] large class of natural motions [is formed by] the natural motions of the elements. [...] the elements move to their natural places — the light bodies up and the heavy ones down — by an appeal to their respective natures as causes (“that it is simply their nature to move somewhere, and this is what it is to be light and to be heavy,” Physics Book VIII, 255b13–17).
See Aristotle's text :
[Phys, 255b] The activity of lightness consists in the light thing being in a certain place, namely high up: when it is in the contrary place, it is being prevented. The case is similar also in regard to quantity and quality. But, be it noted, this is the question we are trying to answer — how can we account for the motion of light things and heavy things to their proper places? The reason for it is that they have a natural tendency towards a certain position; and this is what it is to be light or heavy, the former being determined by an upward, the latter by a downward, tendency.
The aristotelian theory affirmed that there was a natural 'affinity' between substances that were alike. Thus, a stone fell towards the ground because of a natural, telluric affinity, and fire/smoke rose to the sky because of a natural affinity, too...
How was gravity explained in Ancient Greek and Roman times?
It wasn't.
As xxavier and Mauro say, people like Aristotle claimed that things had a natural affinity or tendency towards a certain location. That's no explanation at all. That's what's missing from the Wikipedia article sempaiscuba was referring to.
The Wikipedia article does include a section on general relativity which purports to explain gravity. However it doesn't. Note this: "Einstein proposed that spacetime is curved by matter, and that free-falling objects are moving along locally straight paths in curved spacetime". He didn't, because:
Spacetime curvature is associated with the tidal force, not the force of gravity.
Spacetime models space at all times, so there's no motion through spacetime.
A concentration of energy causes spacetime curvature not just matter.
What Einstein did say is this: “As a simple geometric consideration shows, the curvature of light rays occurs only in spaces where the speed of light is spatially variable”. Unfortunately you tend not to hear about this, which means gravity is generally not explained in modern times either.
Gravity is of course something that we can all observe. Stuff falls towards the ground. But not everything: some things like steam or smoke defy this force and instead float up.
However they don't do this in a vacuum. Note that the Wikipedia vacuum article says this: "Vacuum has been a frequent topic of philosophical debate since ancient Greek times, but was not studied empirically until the 17th century". The ancient Greeks weren't big on experiment.
