The position many scientists take on the question of extraterrestrial life is this: the Galaxy contains tens of thousands of life-bearing planets, and on some of those planets ETCs exist that are technologically far in advance of our own. This conclusion seems to follow from the Principle of Mediocrity — the notion that Earth is a typical planet orbiting a common type of star in an ordinary part of the Galaxy. The principle has served science well since the time of Copernicus. Scientists who take this position, however, have to answer Fermi's question. If ETCs exist, then why are they not here? At the very least, why have we not heard from them?
There are a variety of answers, ranging from the technological (interstellar travel is difficult to achieve, for example) to the sociological (for example, all societies sufficiently advanced to develop interstellar travel inevitably destroy themselves). One weakness of many of these answers, particularly sociological answers, is that to explain the Fermi paradox they must apply to every ETC. I leave the reader to decide whether such answers can resolve the paradox, either singly or in combination.
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