Life is an event which is intrinsically non-deductible from first principles. This idea, in a different context, has been claimed and argued by the biologist and Nobel laureate Jacques Monod in a book published in 1970 (Monod 1970).
On the grounds that life can neither be denied nor fully predicted, a further step follows, namely, the declaration of the existence of extraterrestrial life as a principle of Nature.
The main advantages of a principle for the existence of extraterrestrial life are: (i) the solution of paradox-like statements concerning extraterrestrial life (e.g., Fermi's question), (ii) the suppression of geo- and anthropocentric ideas, and (iii) the creation of a logical basis for future theoretical and experimental investigations. In practice the latter means that one does not need to justify any scientific project on extraterrestrial life searches (for example, Sagan and Drake 1975) regarding its logical foundations: the principle provides (is) the foundation.
The crucial experiment for the origin of life has not to be done; it was already done on Earth. It seems fair to believe that given a set of yet unknown environmental conditions life is bound to flourish. Examples of such a conception, i.e., that life is not a privilege of our local environment, are multiple in the literature, from the early incursions by Giordano Bruno (e.g., Gatti 1999) and Christiaan Huygens (1798) through the modern ages with Robert Goddard (see Sagan 1979), Sagan and Salpeter (1976), and others.
It is worthwhile pointing out that the meaning of life used here is definitely not restricted to carbon-based organisms developed upon watery substrates. A broader concept is envisaged, which is not new and may be found, for example, in the investigations by Sagan and Salpeter (1976) of a possible Jovian ecology, or in the literary speculations of a living interstellar cloud, by Hoyle (1957), and of a structured cometary mind, by MacLeod (2000) -- incidentally, both likely being fed by some sort of ubiquitous cosmic plankton.
Asserting the precise meaning of life is otherwise beyond the scope of the present note; the reader is referred to the above-mentioned book by Monod for a thorough discussion on the definition of a living organism.