In 1964, astronomer Nikolai Kardashev proposed that ET societies fall into three fundamental categories, each based on environmental resourcefulness. A hypothetical "Type I" civilization, for example, effectively conquers all available resources on its home planet (and, just as importantly, fails to destroy itself in the process). A "Type II" civilization is more robust, utilizing the resources of its solar system. Even more daunting, a "Type III" civilization is characterized by its ability to harvest energy on a galactic scale.
If a Type I or Type II civilization seems godlike to our own relatively primitive "Type O" civilization, it's worth remembering that even a solar system-spanning intelligence is far from immortal. But destroying a Type III civilization would prove considerably more difficult. Having inundated space and assumed control of millions of stars, a Type III civilization would be able to anticipate celestial mishaps and perhaps even prevent them.
The "Kardashev Scale" has become a mainstay among futurists seeking to plot humanity's own future. But while not without its usefulness, Kardashev's model remains speculative. There's no guarantee that a high-technology ET civilization will abide by his template, however sensible it might seem. The Kardashev Scale assumes, for instance, that aliens will share our own imperialistic sensibility. In truth, they might be far less aggressive, requiring less energy than we might expect; there's no readily apparent reason why even the most resplendent of civilizations would require the resources of an entire galaxy.
One can think of any number of activities that might engage ET societies; our evident failure to observe Type III civilizations is hardly proof that ETs don't exist. Ultimately, the Kardashev Scale serves as an engaging speculative exercise. Unfortunately, like the Fermi Paradox, it's evolved into a sort of cosmic doctrine, eagerly defended by pundits who seem genuinely incapable of realizing its anthropocentric limitations.
This piece originally appeared at aboutSETI.com.