Abstract

Discussion of the extraterrestrial hypothesis (ETH), the hypothesis that an extraterrestrial civilization (ETC) is active on Earth today, is taboo in academia, but the assumptions behind this taboo are faulty. Advances in biology have rendered the notion that complex life is rare in our Galaxy improbable. The objection that no ETC would come to Earth to hide from us does not consider all possible alien motives or means. For an advanced ETC, the convergent instrumental goals of all rational agents – self-preservation and the acquisition of resources – would support the objectives of removing existential threats and gathering strategic and non-strategic information. It could advance these objectives by proactively gathering information about and from inhabited planets, concealing itself while doing so, and terminating potential rivals before they become too dangerous. Other hypotheses of ETC behavior, including the zoo/interdict hypothesis and the dark forest hypothesis, also undercut the objection that the ETH is highly improbable. The ETH does not require support from extraordinary evidence because the presence of an ETC on Earth is not highly unlikely and would overturn none of our well-tested scientific knowledge. The fact that most reports of unidentified anomalous phenomena (UAP) have natural or human explanations does not count against it. Inference to the best explanation offers a way to find evidence for this hypothesis, and some evidence for it exists, some of it taking the form of reliable witness reports. The most plausible alternative explanation for some UAP reports declines in probability over time. A hypothesis that is not highly improbable, does not contradict any well-established facts or theories, and explains otherwise unexplained evidence is a rational hypothesis. Since the ETH is a rational hypothesis, investigation of it should not be taboo.