A deep problem—philosophical rather than factual—stymies all our attempts to define the nature of life. Scientific generalizations require replication, the demonstration that a given set of forces and substances will yield the same result when brought together under the same conditions. Ideally, we test for replication with time-honored procedures that scientists call controlled experiments—artificially simplified situations manipulated by human observers to guarantee (within the best of our ability) an exact repetition of all timings, forces and substances. If we achieve the same result in each of several replications, we then gain confidence that we may be witnessing a predictable generality based upon a law of nature. This search for replicates underlies the efforts—and partial successes—of scientists to synthesize living matter from the presumed chemical constituents in the "primordial soup" of the earth's original oceans. Can we create some rudimentary forms of life by exposing these constituents to known sources of energy (lightning from electrical storms, heat from oceanic vents, for example) under the presumed conditions of the earth's early atmosphere and surface?

In this context of accepted scientific procedures, single occurrences present a knotty problem. Their "truth" cannot be denied, but how can we use their existence to assert any generality rather than an explanation for a singular circumstance? For specific events of history—the rise, domination and extinction of dinosaurs, for example—we seek no such generality, and specific narrations for bounded events supply the explanations we seek. Thus a particular asteroid, striking the earth 65 million years ago and leaving evidence of its impact off the Yucatan Peninsula, probably triggered a global extinction that sealed the fate of dinosaurs and many other creatures. In developing such evidence, we have explained a unique historical event, but we have not discovered a general law of nature.

But when we ask questions about the nature of life—when we wonder, for example, how common life may be in the universe or inquire whether any potential life on other worlds will look like the life we already know on Earth—then we are seeking to understand general principles about the essential character of the natural universe and not simply to explicate a particular set of historical events. To formulate such general principles, as I argued above, we need replicates, either made in our laboratories or found elsewhere in the universe.

The life that we know, however wondrous in extent and variety, all proceeds—or so our best inferences tell us—from one single experiment. The biochemical features underlying this amazing variety and the coherent fossil record of 3.5 billion years (implying a single branching tree of earthly life with a common trunk) indicate that every living thing on Earth, from the tiniest bacterium on the ocean floor to the highest albatross that ever flew in the sky, arose as the magnificently diversified evolutionary outcome of one single experiment performed by nature, one origin of life in the early history of one particular planet.

Thus we can define the life we know by specifying its common features and properties. But if we wish to move from this knowledge to statements about the general nature of any potential life in the universe, we remain stymied in two key ways.

First, we can make no reasoned conjecture about the frequency, or even the existence, of life elsewhere in the universe. As an optimist by temperament and as a betting man, I allow that certain features of the natural world would lead me to place my chips on yes if someone forced me to wager. But I also know the difference between a pure flutter based on hope and a smart play based on genuine probabilities.

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