Introduction

Speculation on life on other worlds goes right back to Greek times.  Around 400 B.C Epicurus wrote to Herodotus:

"There are infinite worlds both like and unlike this world of ours.........we must believe that in all worlds there are living creatures and plants and other things we see in this world." Aristotle, however rejected this concept outright, believing that: "The world must be unique, there cannot be several worlds"

As we shall see, this polarisation continues to this day.

The Roman poet and philosopher Lucretius also professed a faith in the plurality of worlds.
The Copernican Revolution lent observational credence to this idea.
When Galileo turned the first telescope on the heavens in 1609 he discovered that:

"The galaxy is nothing other than a mass of innumerable stars gathered together in clusters".

He told Milton, who described the Milky Way as such in Paradise Lost. This confirmed an hypothesis first put forward by Democritus in 400 BC.
Giordano Bruno maintained that both the stars and the planets were inhabited (De l’infinito Universo e Mondi 1584). Johannes Kepler speculated on the Moon’s inhabitants and the C17th Dutch astronomer Christiaan Huygens let his imagination run riot in his treatise Cosmotheros where he describes inhabitants of all the known planets. Even the great C^18th philosopher Immanuel Kant wrote extensively on extraterrestrial inhabitants and devised a scheme that related the nature of these beings to their distance from the centre of the Universe.

The general trend has been from philosophical and theological enquiry to scientific argument backed up by empirical facts. The rise of microscopy and later, Darwinism, led to an intellectual climate where noted scientists of all dispositions contributed actively to the debate. In 1907 the Swedish chemist Svante Arrhenius proposed the panspermia hypothesis, whereby life was brought to Earth in the form of extraterrestrial micro-organisms that drifted from world to world, pushed by radiation pressure from the stars.

Current theories and speculations are firmly rooted in scientific methodology, yet in many areas of knowledge there is much to be discovered before any definite answers can be attained.

One of the crucial questions to answer before looking for life on other worlds is "What actually is life?" and "How does life start?"