Extraterrestrial life

       Thousands of years ago, the idea that the planets were populated by intelligent beings was uncommon. The idea was that the planets themselves were intelligent beings. Mars was the god of war, Venus was the goddess of beauty, Jupiter was the king of the gods. In early Roman times a few writers, for example Lucian of Samasota, conceived that at least the Moon was a place that was populated as the Earth was. His science-fiction story describing travel to the Moon was called the "True History." It was, of course, false. The idea of the planets as an elegant celestial clockwork created by the Deity for the amazement and utility of men emerged in the Renaissance. 
         In the year 1600 Giordano Bruno was burned to death at the stake, in part for uttering and publishing the heresy that there were other worlds and other beings inhabiting them. The pendulum swung far in the other direction in subsequent centuries. Writers such as Bernard de Fontenelle, Emanuel Swedenborg, and even Immanuel Kant and Johannes Kepler could safely imagine that perhaps all the planets were inhabited. Indeed, the idea was expressed that the name of the planet gave some hint to the character of its inhabitants. The denizens of Venus were amorous; those of Mars, warlike or martial; the inhabitants of Mercury, fickle or mercurial; those of Jupiter, jolly or jovial. And so on. The great British astronomer William Herschel even supposed that the Sun was inhabited. But as the extremes of the physical environments in the Solar System became clearer and the exquisite adaptation to the environment of organisms on Earth became more apparent, skeptics arose. Perhaps Mars and Venus were inhabited, but surely not Mercury, not the Moon, not Jupiter. And so on. In the last few decades of the nineteenth century the observations of the planet Mars by Giovanni Schiaparelli and Percival Lowell quickened public excitementabout the possibility of intelligence on our planetary neighbor. 

Aristotle - Physics

           When the objects of an inquiry, in any department, have principles, conditions, or elements, it is through acquaintance with these that knowledge, that is to say scientific knowledge, is attained. For we do not think that we know a thing until we are acquainted with its primary conditions or first principles, and have carried our analysis as far as its simplest elements. Plainly therefore in the science of Nature, as in other branches of study, our first task will be to try to determine what relates to its principles. 
         The natural way of doing this is to start from the things which are more knowable and obvious to us and proceed towards those which are clearer and more knowable by nature; for the same things are not 'knowable relatively to us' and 'knowable' without qualification. So in the present inquiry we must follow this method and advance from what is more obscure by nature, but clearer to us, towards what is more clear and more knowable by nature.Now what is to us plain and obvious at first is rather confused masses, the elements and principles of which become known to us later by analysis. Thus we must advance from generalities to particulars; for it is a whole that is best known to sense-perception, and a generality is a kind of whole, comprehending many things within it, like parts. Much the same thing happens in the relation of the name to the formula. A name, e.g. 'round', means vaguely a sort of whole: its definition analyses this into its particular senses. Similarly a child begins by calling all men 'father', and all women 'mother', but later on distinguishes each of them.