Instigator / Pro

Extraterrestrial Intelligence is More Likely Common than Uncommon


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With 4 votes and 6 points ahead, the winner is ...

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Two weeks
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Open voting
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Two weeks
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Four points
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~ 1,215 / 5,000

(this is my first debate on this site, so my apologies if my description does not fully conform to the usual expectations)

Pro will argue that, on balance, the commonality of extraterrestrial intelligence is more likely than not.

Con will argue that, on balance, the rarity of extraterrestrial intelligence is more likely than not.

R1 Introductory arguments
R2 Response to R1 arguments
R3 Response to R2 arguments and concluding statements (no new arguments may be introduced this round)


Extraterrestrial: Life forms that do not originate from Earth.

Intelligence is hard to define. I suggest my opponent and I refer to the following definition - "Intelligence is a very general mental capability that, among other things, involves the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly and learn from experience." ( However, I accept that my opponent may wish to offer a different interpretation. So long as doing so is germane to our arguments, I think that is fine. In that case, we may have to debate the reasonableness of our competing definitions of intelligence.

Round 1
Thanks to Ramshutu for accepting. I look forward to an interesting debate.

I will be drawing on the Drake Equation for my opening argument (I encourage those unfamiliar to click the link).


There are an estimated 160 billion exoplanets in our galaxy.

According to recent preliminary analysis of the ESA's Gaia satellite, 2-12% of exoplanets may be habitable. This may seem small at first, but 160 billion x .02 is 3.2 billion. Not bad.

The next phase of the Drake Equation asks us to consider the likelihood of life arising on a habitable exoplanet. Given that life on Earth arose in less than 1 billion years of Earth's existence, it is reasonable to conclude that the development of life (biogenesis) is fairly easy. S. Blair Hedges of Temple University states, "If life arose relatively quickly on Earth, then it could be could be common in the universe." Though I would be confident at placing the likelihood of life arising on habitable planets near 100%, I will be conservative and posit 20%. This leaves us with 640 million planets harboring life.

Next, we must ask what fraction of life-bearing worlds will produce intelligent life. Again, I turn to Earth. The first human (homo erectus) dates back 1.89 million years. Thus, human intelligence essentially sprung up after 3.5 billion years of life on Earth. Since the jump from life to intelligent life took significantly longer than the occurrence of biogenesis, I will be more conservative and posit that of the planets that produce life, only 1% produce intelligent life. I think this is very generous, given that 3.5 billion years is small in cosmic terms. With this 1% figure, 6.4 million worlds home to intelligent life remain in the galaxy. By any reasonable metric, we should consider 6.4 million to indicate extraterrestrial intelligence's commonality.

I am not claiming 6.4 million intelligent species as hard truth. I have made rational guesses using available data. My job is not to definitively prove commonality. I merely need to argue commonality is more likely than not. I believe I have done so.

Finally, I should address the obvious Earth-centrism of my data for habitability, biogenesis, and development of intelligence. Some might consider this a weakness, but it could just as well be a strength. Earth-centrism could just as well be a conservative approach to estimating the abundance of extraterrestrial intelligence, insofar as it excludes non-Earth-like circumstances from its consideration.
Firstly, I would like to thank my opponent for this debate, and look forward to the discussion!

0.) Definition and Burden.

Pro must show that the intelligent life is likely common. This is more than simply showing the possibility that life is common.

“Common” - Given the size of the galaxy, commonality would imply that there are at least tens of thousands of intelligent civilizations in the hundreds of millions of planets that likely exist.

“Intelligent” I propose that for this debate we talk about a level of intelligence capable of building a civilization.

1.) Likely uncommon.

The negative case can be summed up as follows:

P1: if intelligent life is common, the galaxy should be full of detectable intelligent civilizations.
P2: We have not observed any intelligent civilizations other than our own.
C1: intelligent life is uncommon.

Or, summarized by Enrico Fermi:
“Where is everyone?”

Defense of P1: 

Due to the scale of the galaxy, even if intelligent life is unlikely, the Drake equation implies that the galaxy should be teeming with technologically advanced, intelligent civilizations, of which many should pre-date our own by thousands or even millions of years.[1]

Defence of P2: 

We have been observing the galaxy for decades with radio telescopes looking for observational evidence of intelligent life. We have detected no radio broadcasts or any evidence indicating intelligent civilizations[2],  there is also no credible evidence that we have ever been visited other than conspiracy theories and outlandish claims on the history channel.[3]


Given the premises are both proven - the conclusion follows.

The Drake equation presupposes the likelihood of intelligent life using a biased sample size of 1. Observational evidence refutes the validity of the equation - with the most likely explanation of the lack of evidence is that intelligent life is rare enough that we haven’t yet met it.


Round 2
Thanks to Ramshutu for their reply.


"Pro must show that the intelligent life is likely common. This is more than simply showing the possibility that life is common."
I agree, so long as we interpret "likely" as "more likely than not" AND recognize Con carries a similar burden. I insist on this standard because it is in the debate description.

"“Intelligent” I propose that for this debate we talk about a level of intelligence capable of building a civilization."
I accept, for simplicity's sake.


The remainder of my rebuttal round will be devoted to providing solutions to the Fermi Paradox (FP).

So, if intelligent species (IS) are common, why haven't we made contact?

1. Many IS may not develop sufficiently advanced technology to communicate.

2. Many IS may use communication technologies we do not possess.

3. Many IS may listen rather than signal. We do this. Alexander Zaitsev calls our neglect of sending signals while listening for signals the SETI Paradox. Citing the SETI 2020 roadmap, Zaitsev notes that our signaling-to-searching ratio is less than 1%. If this is a common behavior among intelligent species, establishing contact would be difficult.

4. Many technologically advanced IS may withdraw from reality and 'plug in' to virtual reality, thus curtailing/eliminating communication attempts.

5. Human existence is brief on the cosmic time scale (clip from Carl Sagan for perspective). We have only been engaged in SETI for 60 years. Given radio signals are limited to the speed of light, and given the galaxy is an estimated 100,000 light-years across, should it really be surprising we've received no contact yet? Our 60 years of searching is puny compared to the vastness of the cosmos.


I do not claim any single solution resolves the FP on its own. Rather, I believe their cumulative weight shows there is no contradiction between the notion of plentiful extraterrestrial intelligence and no observation (as of yet). There are reasons why communication might be impossible (#1-4), and even if it was not in some cases, the vastness of time and space are no small obstacles (#5).

In Round 1 I used the Drake Equation, available evidence, and reasoning to demonstrate the likelihood of extraterrestrial intelligence. In Round 2, I have offered solutions to the FP that, taken together, give us good reason to dismiss the FP's status as a real paradox. For these reasons, I affirm my position: extraterrestrial intelligence is more likely common than uncommon.
I apologize to my opponent for the delay - real life is being problematic! I will dive right in:

R1.) Potential solutions for the Fermi Paradox.

Pro is undone by the same law of large numbers that he uses to support the Drake equation.

My opponent discusses several speculative solutions to the Fermi paradox. Let’s start with my opponents 6.4m worlds.

Let’s assume 60% are more primitive than us - 2.56m

Say 50% of those don’t explore or transmit outside their planet - 1.28m.

Of the rest, 90% develop technology we cannot sense. - 128k.

Even if only 0.1% of the remaining species are too far away or simply by blind luck haven’t had an antenna or telescope pointed anywhere near them: we should still have definitively observed at least 128 alien civilizations.

R2. Alternative explanations 

Even considering my opponents speculative numbers and explanations - we should still have observed multiple intelligent species.

The alternative explanation to why we haven’t is simple; the assumption that intelligent life is common is based on what happened on earth not being lucky.

My opponent uses the probability of life occurring on a habitual planet at 20%, that it becomes intelligent at some point at 1%.

If the chances of life in our timescale is only 2%, complex life is 10%, and intelligent life from that that only 0.1% - it cuts down the number of IS from 6.4m to 6400, under the threshold established for commonality.

This is not even counting the potential for intelligence to arise, then destroy itself through nuclear weapons or climate change - or simply go extinct like 99% of all species. Humanity may not exist in another 1m years due to our own actions or an extinction event that kills us all; such events may not be uncommon on other planets, lowering the number of civilizations.


The only fact we have is that despite substantial observation of a galaxy that should be teeming with intergalactic civilizations - we have never seen it.

As shown,the simplest and least speculative explanation is that it is not teeming with intelligent life as such life is simply a little rarer than my opponent states.

Round 3
I must note my opponent violated round structure rules. In the description I indicated R2 arguments should be used to reply to R1 arguments. But in his R2, Ramshutu offered a rebuttal to my R2 argument. I politely request voters consider docking Ramshutu for conduct.

Re: Solutions to Fermi Paradox (FP)
Ramshutu takes the solutions I offered in R2 and applies filters to rebut me. To reply effectively, I think I really only need to focus on the filter he applies to solution #5 (obstacle of space/time). He says even if 1 in 1000 species we could communicate with were within our range, we should have made contact by now.

At first, 1 in 1000 seems conservative. But the galaxy is 100,000 light years across. SETI is only 60 years old. Per my R1 numbers, 1 in every 25,000 planets host intelligent life. Within 50 light years of Earth, there are only 1800 stars. Even in a galaxy full of intelligent life, the speed of light would likely prevent us from making contact in a short 60 year window.

Re: Alternative Explanations and Conclusion
If the chances of life in our timescale is only 2%, complex life is 10%, and intelligent life from that that only 0.1%
I am especially opposed to the section in bold. As noted in R1, life arose on Earth nearly instantaneously. This leaves us with two possibilities: either the swift development of life on Earth represents a cosmic fluke, or the development of life is easy (suggesting its universal abundance). In the absence of hard evidence, I submit it is most reasonable to favor the latter possibility. For this reason, I maintain that my 20% estimate provided in R1 is conservative. If we use my generously conservative 20% and keep the rest of Ramshutu's numbers in the quote, we arrive at 640,000 intelligent species.

As shown,the simplest and least speculative explanation is that it is not teeming with intelligent life
On speculation: It's the nature of this topic. To the extent that I must be criticized for being speculative, so must my opponent.

On simplicity: intelligence's rarity is conceptually no simpler than intelligence's commonality. In both cases, innumerable variables are involved.

In R1, I used available evidence and reasoning to conclude alien intelligence is likely common. In R2 and R3, I fended off attempts to undermine this proposition with the FP and alternative filters. As I have defeated these objections, and as my R1 is the most reasonable assessment of the available data, I urge judges to vote Pro.