In 1975, Hart published his paper “An Explanation for the Absence of Extraterrestrials on Earth,” where he methodically presents every well-known explanation for this (below) and then presents his own explanation: that mankind on our Earth is the first civilization in our Galaxy.
First, let me say that I appreciate this paper. I did not find any logical fallacies or leaps in an attempt to justify what he claimed. Instead, everything seemed well thought-out and matched with diction appropriate to his stance: that given the paucity of evidence and lack of other theories that sufficiently support this paucity, we should accept the last explanation (that we are the first).
As mentioned before, Hart methodically goes through possible explanations for why there are currently no creatures from other planets on Earth (he does mention that this could be wrong, but is improbable). The top explanations for this include physical explanations (space travel is infeasible), sociological explanations (the culture of the civilization does not want to visit other civilizations), temporal explanations (the civilizations will visit us but have yet to arrive because space travel takes time), and simply that others have visited us before but decided not to stay. Each of these theories is introduced, explained, and then debunked. As a spoiler alert, any explanation of why there are currently no alien species on Earth would have to hold for all civilizations that arise, and for the lifetime of these civilizations.
Space travel is in fact possible. This paper was published after the moon landing, so even at that time, Hart knew that humans could at least go to the moon. Any problems dealing with space travel can be overcome with technological advances or even just enough time and money.
Hart sufficiently (in my mind) eliminated sociological explanations completely. The unwillingness to explore or visit places (something not really seen in our society today) could be derived from culture or religion, but it is something that is most likely fluid with time; this unwillingness would be challenged over time with new generations. Even if thousands and thousands of generations of a civilization are in fact against visiting others, this outlook could not possibly be held (for forever) by every single civilization.
Temporal explanations are also dismissed quickly (dismissed as unlikely but plausible); even though space travel takes time, it would take us only 650,000 years to traverse the Galaxy. For this explanation to satisfy the fact that no alien species are currently on Earth, the first civilization with the desire and ability to travel space would have required 10 Gyr to arise AND for our civilization to arise as second about 1Myr later. Because of the unlikeliness of both of these being true, Hart dismisses this argument as possible, but highly unlikely.
Lastly, the theory that other species did visit but are no longer here is dismissed in the same why sociological explanations were eliminated. Although plausible for this to be true for one civilization, it is unlikely that all civilizations that visited decided to leave and never return (or died here, or died before being able to return).
Since there is no adequate explanation for why there are currently no alien species on Earth, Hart concludes that we are probably the first civilization in our Galaxy.
I personally enjoyed this paper mostly due to the lack of logical leaps and bounds. That being said, I think the conclusion is a bit pessimistic. Since we can’t really prove a negative (that there are no other civilizations) and since finding intelligent life would be amazing, we might as well keep looking! Hart includes in this conclusion that “an extensive search for radio messages from other civilizations is probably a waste of time and money,” which I completely disagree with (I might be biased). I think there is merit to these searches beyond just looking for aliens; they lead to valuable data and technologies, and even lead to some people joining the fields of astronomy and astrobiology.