Argument: Extraterrestrials might prove unintelligible, or even hostile toward humanity:

Were extraterrestrials to actually be contacted, an unlikely event in itself, the ability to share in any form of meaningful communication might well prove more difficult than might first seem. The extraterrestrials will have evolved on a different world, perhaps along lines so alien to that of Earth's life that it would be utterly impossible to understand what they said, even if they could be heard. Language is built largely on frames of reference, and when an extraterrestrial, evolving on an entirely alien world, perhaps possessing an entirely alien thought process, and certainly having an entirely alien linguistic frame of reference, no level of communication might be possible. With no conceptual similarities or cognitive common ground, the potential for sharing knowledge between species would likely prove impossible to decipher. They could simply be too alien. Furthermore, the hope that technologically advanced extraterrestrials would be friendly towards humanity is entirely unfounded. They might well prove hostile, and if they are more technologically advanced, they could prove an existential threat to the survival of mankind1. This is especially true if the extraterrestrials were actually capable of physically reaching Earth, which would require an understanding of physics and engineering centuries ahead of that of mankind. Welcoming such creatures to Earth could well spell ruin for humanity, as so advanced a civilization might well consider humanity to be of no higher an order of life than we might consider insects. It is far better than humanity not make such a habit of broadcasting its position. It could prove dangerous, especially if the Universe is not as friendly a place as scientists hope.

1 Brin, David. 2006. "Shouting at the Cosmos". Lifeboat Foundation.

Candidate 1: "The potential for sharing of knowledge, were extraterrestrials to actually be contacted, might well be more limited and difficult than might first seem. The extraterrestrials will have evolved on a different world, perhaps along lines so alien to that of Earth's life that it would be utterly impossible to understand what they would have to say, even if they could be heard. They could simply be too alien. There is no reason to believe that any aliens are going to benevolent they could very well be hostile. (Brin, 2006) We should not therefore be attempting to contact aliens to share technology rather we should be focusing on advancing our own technology. 1 Brin, David. 2006. "Shouting at the Cosmos". Lifeboat Foundation."
Candidate 2: "It seems unlikely that humans would not be able to survive a great change of circumstance. The human mind is quite resilient, and while the change in how people viewed themselves and the Universe would likely change substantially, it might well be for the better. In fact, discovery of extraterrestrials might help free people from the shackles of dogmatic religious belief, which has caused untold suffering to many through the ages."
Candidate 3: "There is no need for us to be searching for alien life to be exploring the universe. The dream of contacting aliens seems to be an example of humanity's dreams gone awry. Earth's beamed messages and questing probes have for their efforts received no return signal. Surely if intelligent life were near enough to have received our signals, they would have replied. Alternatively, if they are too far away, there is no physical means by which they could ever reach Earth, due to the constraints of the laws of physics, which deny objects the power of reaching the speed of light. Furthermore, the value of the drive to explore itself should be questioned, since humanity's track record on the subject is far from stellar; the abysmal treatment of the Native Americans by European settlers, for example, shows that the will to explore can come with terrible costs."
Candidate 4: "Just because efforts to contact extraterrestrials to date have proven unsuccessful does not mean they are not out there. In fact, as communications technology develops over time, humans' ability to project messages and to receive them will increase manifold. Extraterrestrial transmissions could well be beaming toward Earth, but humans might simply lack the capacity to receive them. There could be an interplanetary conversation happening right now, and no one would even know (Sagan, 1973). Only by pushing the boundaries of science and of human imagination can such technological improvements arise. The paradigm of exploration must be maintained with the scouring for life amongst the heavens."

It seems unlikely that humans would not be able to survive a great change of circumstance. The human mind is quite resilient, and while the change in how people viewed themselves and the Universe would likely change substantially, it might well be for the better. In fact, discovery of extraterrestrials might help free people from the shackles of dogmatic religious belief, which has caused untold suffering to many through the ages.
