Oh how grassy is this hopper,
How this berry ripely rasps.
I would never have conceived it
If I weren’t conceived myself!
—Wisława Szymborska (1923-2012)
Astrobiology is sometimes and with some justice disparaged as a science with only one data point. This is not to deny the wealth of geological, astronomical and biological knowledge that can now be brought to bear on astrobiological questions, or the important progress we have made in working out where life as we know it could find safe harbour in the universe. Rather, the point of the objection is to underscore that notorious qualifier: as we know it. Is the Earth’s watery, carbon-based biosphere unique? Is it typical of Earth-like planets? Or is it only one among many different forms of self-organized complexity scattered through the cosmos? These questions are still beyond the reach of our science.
Perhaps there are gaps in our understanding as profound and significant as if we had never encountered biology at all. Indeed, if we hadn’t already discovered life on Earth, we’d never have realised it was possible. Suppose you had an up-to-date knowledge of fundamental physics, chemistry, astronomy and geology but no prior acquaintance with biology or biochemistry (including your own; perhaps you are a disembodied mind). Would you notice a priori the capacity of carbon chemistry to achieve the complexity of a living thing? Would any amount of armchair speculation (or computer modelling) lead you to postulate the existence of a blade of grass? A bacterium?
This may be the scale of our ignorance with respect to an untold number of complex and interesting things in the universe. We are peering now into the great Rumsfeldian hinterland of the “unknown unknowns”. There are more than a hundred billion galaxies in the universe, each with typically billions of stars. Most stars have planets. No two planets are alike even in our own neighbourhood and we have scarcely begun to sample their true variety.
In these circumstances, it is not surprising that some astrobiologists detect a natural kinship between our discipline and philosophy (I am thinking in particular of David Grinspoon and Milan Ćirković). Philosophers—experts in reasoning—have brought clarity and insightful analysis to many other speculative and conceptually treacherous branches of science as well as to our understanding of scientific methodology in general. In astrobiology, the philosopher Carol Cleland has done important work on the (lack of a) definition of “life” and has outlined the possibility of a “shadow biosphere” on Earth.
Bertrand Russell claimed that:
Philosophy, though unable to tell us with certainty what is the true answer to the doubts which it raises, is able to suggest many possibilities which enlarge our thoughts and free them from the tyranny of custom. Thus, while diminishing our feeling of certainty as to what things are, it greatly increases our knowledge as to what they may be.
Astrobiologists are unusual among scientists because they are searching for just this sort of knowledge: not yet of what things are but of what they may possibly turn out to be. Our speculation must be constrained by philosophical and scientific rigour. But we may not know what we are looking for until we find it.