"I hope that our Mars probes discover nothing. It would be good news if we find Mars to be sterile. Dead rocks and lifeless sands would lift my spirit."
His reasoning is based on two extrapolations from objective observation. First, there are probably many billions of Earth-like planets in the universe. This assumption is an extrapolation from peer-reviewed observation of many billions of galaxies in our universe, many billions of star systems in our galaxy, and many large planets in star systems near us. Second, there are probably no advanced civilizations in the universe. This assumption is an extrapolation from peer-reviewed observation of a decades-long lack of phenomena, such as signals or spacecrafts, associated with advanced civilizations. I'll call these "Fermi assumptions".
Given the Fermi assumptions, Bostrom points out that there must be a reason (or set of reasons) for which the many billions of Earth-like planets have produced no advanced civilizations. He labels that reason the "Great Filter", and notes that it may be behind or ahead of us in the timeline of human civilization. Candidates for filters behind us would be extremely low probability events in our past development (maybe the emergence of basic life, its formation into prokaryotic and eukaryotic cells, or their organization into multi-celled organisms -- subsequent events in our development are relatively probable). Candidates for filters ahead of us would be extremely high probability events in our future development (maybe technology-enabled destruction through war or environmental disaster).
From there, Bostrom reasons, if we begin to find independent sources of life simpler than our own then the Great Filter is almost certainly ahead of us because such discoveries would substantially increase the probability of human-like civilizations without substantially increasing the probability of advanced civilizations. Thus, finding simple life on Mars would be horrible news for the future of human civilization.
If the Great Filter is still ahead of us then "Great Wall" is almost certainly a more appropriate label, given the Fermi assumptions. Without a filter behind us, many billions of other Earth-like planets probably have been home to human-like civilizations, yet none has become an advanced civilization. Given many billions of successes at or near our present degree of civilization and no successes among billions of attempts beyond our present degree of civilization, why should we suppose that we'll become the first advanced civilization?
Further consistent extrapolation from the Fermi assumptions implies that if the Great Filter isn't already behind us then the Great Wall, not only practically inevitable, may be near at hand. Our civilization already produces phenomena that other human-like civilizations could observe, and we've been doing this for at least several decades. Yet we observe a decades-long lack of such phenomena coming from star systems within several decades' reach. Thus, there must be a reason (or set of reasons) for which any Earth-like planets within this reach have produced no human-like civilizations -- let alone advanced civilizations. As the reach of our phenomena broadens, the number of Earth-like planets within reach increases, and eventually the sample of Earth-like planets within reach of phenomena from human-like civilizations is large enough to assume that either the Great Filter is almost certainly behind us or the Great Wall is almost certainly upon us. While the probability of Earth-like planets is not yet determined, if it is sufficiently high then finding simple life on Mars would be horrible news for the present human civilization -- let alone its future.
However, while I'm inclined to accept the first of the Fermi assumptions, I'm not persuaded that the second of the Fermi assumptions is true. We know too little to entertain confidence in our ability to discern advanced civilizations. Moreover, while I do not think we have sufficient evidence for any objective statement regarding advanced civilizations, it is not merely the recent UFO claims that contend for recognition as subjective experience of advanced civilizations, but rather the ancient and persistent weight of religious tradition, and its purported interactions between the human and the divine, that contends. While some find many religious claims shockingly absurd, appeals to absurdity and weak creative thinking skills are not evidence to the contrary of the religious hypothesis. If advanced civilizations exist, why should we suppose they would interact with us in ways that are only slightly more advanced than our current capacities? Why not suppose that their interactions with us would be, at least for now, indiscernible from magic? Why should we suppose that we would have the anatomical capacity to figure out and comprehend their motives? Why not suppose that their capacities so far exceed ours that they actually are communicating with us in ways we cannot yet understand fully? I'm not suggesting an absolutely omnipotent God or infallible prophets, but I am suggesting that which humans might interpret as an absolutely omnipotent God or infallible prophets.
If the second Fermi assumption is not true then any filters behind or ahead of us are not so great as we must otherwise consider them to be. That would not mean we should assume away or attempt to otherwise diminish the seriousness of future risks. It would mean only that we can have reasonable hope in our ability to manage those risks.
So long as we have objectively observed neither advanced civilizations nor simple life originating from another planet, there are two rational and practical perspectives:
1) We almost certainly are not unique and there almost certainly is no Great Filter (although there may be many serious filters ahead of us).
2) We almost certainly are unique and the Great Filter almost certainly is behind us (although there may be many serious filters ahead of us).
However, if ever we objectively observe simple life originating from another planet, there will be only one rational and practical perspective, #1 modified to reflect the new observation:
1) We are not unique, but there almost certainly is no Great Filter (although there may be many serious filters ahead of us).
That's not because the following perspective would not be rational:
3) We are not unique, and there almost certainly is a Great Wall.
#3 is no less rational than #1, but it is far less practical than #1. Given objective observation of simple life originating from multiple planets, and assuming we do not desire the end of human civilization, we should doubt the Fermi assumptions. An important class of truths depend on us for their actualization, and the future of our civilization may be one of those truths. If we generally assume we're doomed, we'll generally behave accordingly. If we generally assume we can manage the risks, serious though they may be, we'll likewise generally behave accordingly. Our behavior, in the end, may make the difference.
Whether or not the Mars probe discovers something, I intend to continue to doubt the second Fermi assumption. If the Mars probe discovers something, I wager Bostrom will begin to doubt the second Fermi assumption. Maybe we are not so unique. Maybe we are not so alone. Maybe they are out there, even around us, and we simply have not yet understood.
Joseph Smith, Doctrine and Covenants 88
45 The earth rolls upon her wings, and the sun giveth his light by day, and the moon giveth her light by night, and the stars also give their light, as they roll upon their wings in their glory, in the midst of the power of God.
46 Unto what shall I liken these kingdoms, that ye may understand?
47 Behold, all these are kingdoms, and any man who hath seen any or the least of these hath seen God moving in his majesty and power.
48 I say unto you, he hath seen him; nevertheless, he who came unto his own was not comprehended.
49 The light shineth in darkness, and the darkness comprehendeth it not; nevertheless, the day shall come when you shall comprehend even God, being quickened in him and by him.
50 Then shall ye know that ye have seen me, that I am, and that I am the true light that is in you, and that you are in me; otherwise ye could not abound.