Everything is black. Complete darkness. Tack. Tack. Tack. A faint crack. Tack. Tack. Tack. Through a tiny gap, soft sunbeams are penetrating. Tack. Tack. Tack. The gap becomes bigger, increasingly larger pieces of the former darkness disappear. Tack. Tack. Tack. Eventually the last barriers break, blocking the path to freedom, to a beautiful new world. The slightly confused gaze of a head surrounded by dishevelled feathers fights hard against the bright sunlight. Jerry? World. World? Jerry. Nice that you are there. Clumsily, Jerry stumbles away from the remains of his former home. He didn’t know what adventures the world would hold for him, but Jerry didn’t care. Whatever was coming, he was ready. The next days just flew by. There was so much to discover. He could walk across lush meadows, watch strange creatures, of which one regularly even provided him with food. A huge two-legged creature which in his mind Jerry called Hank. A very nice guy. After all, he supplied Jerry with food. Days became weeks, stretched to months and years. Jerry grew up quickly and developed a magnificent physique. Hank took great care that Jerry always had enough food and the latter took what he could get. Life was good. Jerry assumed that he had apparently been born in the best of all worlds. Where else could he have lived such a carefree life? There was nothing suggesting that this state of eternal bliss would ever change. Jerry did not feel the slightest trace of suspicion when Hank picked him up one sunny winter morning and carried him into a building Jerry had never entered before. Surely Hank just wanted to show him a new treat. Maybe more, better food? Jerry could hardly wait. He didn’t even get sceptical when Hank gently set him down on a massive wooden table, the surface of which was covered with strange furrows. Jerry watched Hank curiously and saw him taking a shiny object off the wall. Slowly Hank stepped back towards Jerry and spoke soothingly at him. He even laid Jerry on his side so he was more comfortable. Hank was just an incredibly nice guy. Life was good. Jerry saw Hank raising his hand holding the shiny object and then – nothing.
It was Christmas time and Jerry would bring a lot of joy to Hank’s family one last time that night. Life was good. For Hank.
The inductive fallacy
What lesson do we draw from Jerry and Hank’s story?
It introduces us to a central principle of scientific research logic:
“Absence of evidence is not the same as evidence of absence.”
A statistical prediction about the further trajectory of Jerry’s life based solely on his past experience and positive development would probably have assured him that everything is fine and he has a wonderful future ahead of him. Those who have read my previous article at my old blog will already be familiar with the name Nassim Nicholas Taleb and the concept of the Black Swan. Based on his argumentation in both The Black Swan and Antifragile: Things That Gain from Disorder is the opening story of Jerry and Hank.
In philosophy of science, this process is also called inductive reasoning. This means that, based on certain premises, a possible general conclusion is derived. Note the use of the term “possible”, because the conclusion does not have to be inevitable.
The most popular version of this problem is about the already mentioned black swan. If we go around the world and every swan we see is a white one, it is obvious that we conclude that all swans are white. However, a single black swan is enough to show us that our general theory of white swans is not as universal as we originally thought. The consideration that we cannot derive general laws on the basis of incomplete information is several centuries old. The Pyrrhonian skeptic Sextus Empiricus wrote about this as early as the second century:
„When they propose to establish the universal from the particulars by means of induction, they will effect this by a review of either all or some of the particulars. But if they review some, the induction will be insecure, since some of the particulars omitted in the induction may contravene the universal; while if they are to review all, they will be toiling at the impossible, since the particulars are infinite and indefinite.”
The Scottish philosopher David Hume put it more simply in A Treatise of Human Nature a few hundred years later:
„There can be no demonstrative arguments to prove, that those instances, of which we have had no experience, resemble those, of which we have had experience.”
If Jerry had enjoyed a philosophy of science education, he might have thought that his life may not be as idyllic as it seems. Not that it would have saved him, but at least he wouldn’t have left the world completely ignorant.
What is Seen and What is Unseen
I have noticed that the logic behind this idea is often overlooked. Even though we probably have more brains than turkeys. I deliberately do not say that it is difficult for people to understand it, because I think the concept is very easy to grasp. However, we quickly tend to think in linear terms. Or to put it in terms of the French philosopher Frédéric Bastiat: We focus more on what we see than what we do not see.
Bastiat illustrated this mechanism with his parable of the broken window:
Have you ever witnessed the anger of the good shopkeeper, James B., when his careless son happened to break a square of glass? If you have been present at such a scene, you will most assuredly bear witness to the fact, that every one of the spectators, were there even thirty of them, by common consent apparently, offered the unfortunate owner this invariable consolation—“It is an ill wind that blows nobody good. Everybody must live, and what would become of the glaziers if panes of glass were never broken?”
Now, this form of condolence contains an entire theory, which it will be well to show up in this simple case, seeing that it is precisely the same as that which, unhappily, regulates the greater part of our economical institutions.
Suppose it cost six francs to repair the damage, and you say that the accident brings six francs to the glazier’s trade—that it encourages that trade to the amount of six francs—I grant it; I have not a word to say against it; you reason justly. The glazier comes, performs his task, receives his six francs, rubs his hands, and, in his heart, blesses the careless child. All this is that which is seen.
But if, on the other hand, you come to the conclusion, as is too often the case, that it is a good thing to break windows, that it causes money to circulate, and that the encouragement of industry in general will be the result of it, you will oblige me to call out, “Stop there! your theory is confined to that which is seen; it takes no account of that which is not seen.”
It is not seen that as our shopkeeper has spent six francs upon one thing, he cannot spend them upon another. It is not seen that if he had not had a window to replace, he would, perhaps, have replaced his old shoes, or added another book to his library. In short, he would have employed his six francs in some way, which this accident has prevented.
Neither is it possible for us to predict all (un)intended consequences, nor can we always make reliable predictions based on past experience. In principle, this phenomenon is not really problematic, provided that we are aware of its existence and its associated limitations. Both the mechanism described by Bastiat and the previously explained inductive reasoning problem are two sides of the same coin.
The uncertainty associated with this can be found in almost every area of human life and it would be easy to go into more economic and political aspects at this point. However, I would like to take this opportunity to illustrate, using a frequently used example, the consequences of equating the absence of evidence with evidence of absence.
How to see the unseen?
Anyone who has ever taken part in the discourse on anthropogenic climate change in recent years will probably be familiar with the statement that 97% of all climate scientists are convinced of the human influence on climate change. In “climate sceptical” circles it has become part of the routinely used toolkit to question this strong consensus. Even though over the years many scientists have regularly explained how this number is achieved, it seems that hardly anyone has come up with the idea of explaining the underlying logical fallacy to which the “skeptics” regularly succumb. I write very deliberately about “skeptics”, because if they were really interested in an intellectually sincere skepticism in the tradition of Sextus Empiricus, David Hume, Karl Popper and others, there would probably be far less reason for this article.
A study by Cook et al. (2013) entitled Quantifying the consensus on anthropogenic global warming in the scientific literature serves as a basis for questioning the 97% consensus. The following statement can now be found within the paper:
“We analyze the evolution of the scientific consensus on anthropogenic global warming (AGW) in the peer-reviewed scientific literature, examining 11 944 climate abstracts from 1991–2011 matching the topics ‘global climate change’ or ‘global warming’.
We find that 66.4% of abstracts expressed no position on AGW, 32.6% endorsed AGW, 0.7% rejected AGW and 0.3% were uncertain about the cause of global warming.
Among abstracts expressing a position on AGW, 97.1% endorsed the consensus position that humans are causing global warming.”
Reading it for the first time, it seems quite understandable why many people are irritated when they hear that 97% of all climate scientists agree that anthropogenic climate change exists, when it is clearly stated here that only 32.6% of the literature examined has taken a confirmatory position. Subsequently, only 97.1% of these 32.6% think that humans influence the climate in a long-lasting way. Obviously, the universally praised consensus of 97% of all climate scientists is a malicious political conspiracy to lead us all into a green eco-dictatorship. If only life were always so simple.
Of course, the authors of the study are aware of this discrepancy, which is why they write at a different passage in the same (!) study:
„Of note is the large proportion of abstracts that state no position on AGW. This result is expected in consensus situations where scientists ‘…generally focus their discussions on questions that are still disputed or unanswered rather than on matters about which everyone agrees’ (Oreskes 2007, p 72). This explanation is also consistent with a description of consensus as a ‘spiral trajectory’ in which ‘initially intense contestation generates rapid settlement and induces a spiral of new questions’ (Shwed and Bearman 2010); the fundamental science of AGW is no longer controversial among the publishing science community and the remaining debate in the field has moved to other topics. This is supported by the fact that more than half of the self-rated endorsement papers did not express a position on AGW in their abstracts.”
Thus the circle closes again. Just because there is no explicit statement about something does not mean that it does not exist. The absence of evidence is not evidence of absence. In studies about evolutionary processes there is probably no explicit mention every time that evolution is a well documented fact and that the earth is not only 6000 years old. Anything else would be highly redundant.
Analogous to the findings from the data of the 2013 study, Cook et al. prepared a follow-up study that explicitly addressed the question of how deviating percentages could also be explained. Not surprisingly, they write in their summary:
„Therefore, it should come as no surprise that the most common argument used in contrarian op-eds about climate change from 2007 to 2010 was that there is no scientific consensus on human-caused global warming (Elsasser and Dunlap 2012, Oreskes and Conway 2011). The generation of climate misinformation persists, with arguments against climate science increasing relative to policy arguments in publications by conservative organisations (Boussalis and Coan 2016).
Consequently, it is important that scientists communicate the overwhelming expert consensus on AGW to the public (Maibach et al 2014, Cook and Jacobs 2014). Explaining the 97% consensus has been observed to increase acceptance of climate change (Lewandowsky et al 2013, Cook and Lewandowsky 2016) with the greatest change among conservatives (Kotcher et al 2014).
From a broader perspective, it doesn’t matter if the consensus number is 90% or 100%. The level of scientific agreement on AGW is overwhelmingly high because the supporting evidence is overwhelmingly strong.“
To avoid confusion at this point, I was not just interested in showing that there is a high consensus among climate scientists about anthropogenic climate change. Above all, I would like to create an awareness that the wording of a statement does not necessarily permit a generalized derivation. We humans have a strong tendency to think in very simple categories. We observe two successive events in time and automatically believe that A necessarily causes B. However, we do not know whether this is the case. But that B can also be a consequence of causes completely unknown to us, can only be recognized if we consciously reflect about it. Unspoken statements do not always imply non-existence. Perhaps it would be time for us to show some humility in public discourse and occasionally become aware of the limits of our own knowledge.
 Taleb, Nassim Nicholas (2007), The Black Swan: The Impact of the Highly Improbable, Random House
 Taleb, Nassim Nicholas (2012), Antifragile: Things That Gain from Disorder. New York: Random House.
 Sextus Empiricus. Outlines of Pyrrhonism, trans. Robert Gregg Bury (Loeb ed.) (London: W. Heinemann, 1933), p. 283.
 Hume, David (1882). Green, T.H.; Grose, T.H. (eds.). A Treatise of Human Nature : Being an Attempt to Introduce the Experimental Method of Reasoning into Moral Subjects & Dialogues Concerning Natural Religion
 Bastiat, Frédéric (1850). That Which Is Seen, and That Which Is Not Seen
 Cook, John; Nuccitelli, Dana; Green, Sarah A.; Winkler, Bärbel; Painting, Rob; Way, Robert; Jacobs, Peter; Skuce, Andrew. Quantifying the consensus on anthropogenic global warming in the scientific literature. 2013 Environ. Res. Lett. 8. Number 2
 Cook, John; Oreskes, Naomi; Doran, Peter T.; Anderegg, William R. L.; Verheggen, Bart; Maibach, Ed W.; Carlton, J. Stuart; Lewandowsky, Stephan; Nuccitelli, Dana; Green, Sarah A.; Winkler, Bärbel; Painting, Rob; Jacobs, Peter; Skuce, Andrew; Rice, Ken. Consensus on consensus: a synthesis of consensus estimates on human-caused global warming. 2016 Environ. Res. Lett. 11. Number 4