ABSTRACT
Plurality of worlds is the term historically used by many cultures for the concept of other worlds beyond the earth. In ancient Greek times this meant a plurality of ordered world systems, referred to as kosmoi. Beginning in the seventeenth century the term came to mean earth-like planets, complete with intelligent inhabitants. In the twentieth century the tradition has become known as the extraterrestrial life debate, a pursuit that biologists have labelled exobiology and astronomers have labelled bioastronomy. Because it is intrinsically difficult to verify the existence of other worlds and extraterrestrial life, the debate has always incorporated a good measure of philosophy. Critics have called it “a science without a subject,” and some even have questioned whether it is a scientific issue at all. Depending on the period under consideration, historians have concluded both that the debate has been conducted primarily in philosophical terms and that the debate has been conducted primarily in scientific terms.
