Reviewed in the United States on February 10, 2018
Harari primarily presents an hypothesis as to how Homo sapiens who successfully migrated out of Africa circa 70 kya (thousand years ago) and thereafter, successfully dominating the planet, differed from earlier Homo sapiens who attempted out migration previously (e.g. around 130 kya) but were not successful. Keep in mind that Homo neanderthals in Europe and Homo erectus in Asia had climate and environment adaptations that sapiens did not have and were separate species. Harari claims (assumes) that the later sapiens differed from the earlier sapiens in only one way. They had evolved the ability to form an "inter-subjective reality," as opposed to objective reality (physical) or subjective reality (in only one person's mind). They were able to tell stories and believe them, independent of physical reality.
Everything from myth to religion to nations to moral codes to money are inter-subjective realities according to Harari. They have force in the physical world as long as people believe them, and cease to exist the moment people no longer believe them. This explains how people could cooperate in groups larger than 150, giving them a military and security advantage, and encouraging specialization which eventually gave them a technological advantage.
Moreover, Harari claims (assumes) all these later sapiens were genetically identical, and that the variations in societies are purely cultural, i.e. inter-subjective realities. He presents history as an erratic evolution toward global unity, which is essentially demanded by the nature of inter-subjective realities, requiring belief of all those in mutual frequent contact, but he doesn't say how.
In fact, Harari presents only anecdotal evidence for his claim. He presents no empirical studies regarding the flexibility of humans toward inter-subjective realities, and no mathematical models of its development, evolutionary advantage, or stability. He describes the scientific method as an important development, and requires it to include both mathematical models and verification of them. But he does not use either in his treatise. Thus he presents an important and interesting hypothesis, but not in a scientific manner. He makes not even suggestions as to how to further formulate or verify it as a scientific theory. Perhaps he is trapped in the inter-subjective reality of history as liberal arts, not science.
The term "intersubjective" does exist in the literature of psychology and philosophy, primarily as a synonym for "agreement," but there is no agreement about its definition (Gillespie and Cornish 2009, Journal for Theory of Social Behavior) state:
"The concept of intersubjectivity is used widely, but with varying meanings. Broadly speaking, we take intersubjectivity to refer to the variety of possible relations between people’s perspectives. If we take social life to be founded on interactions then intersubjectivity should be a core concept for the social sciences in general and understanding social behaviour in particular. Perhaps because of this broad relevancy research has been fragmented and at least six definitions are in circulation. Most simplistically, intersubjectivity has been used
to refer to agreement in the sense of having a shared definition of an object."
The biggest complaint I have about Harari is that he does not distinguish between his opinion and facts, nor explain the background of how he arrived at the theory of inter-subjectivity. The study of the evolution of cooperation is a hot topic, with political scientists, biologists, mathematicians and even physicists all having theories, and much data collected and many math models developed. It is apparent Harari is aware of this, but does not tell us how his theory fits in. I can only conclude he finds his powers of popular persuasion greater than his powers of scientific persuasion and critical analysis, so he writes a long book instead of a focused research paper.
By the way, you can find excellent video summaries and reviews of this book on the web, and even a "summary" for sale as an eBook. I originally got interested from the video summary.
Near the end Harari reports on happiness research. In this section of the book he takes exception to his usual approach, giving us descriptions of studies and names of researchers so we can trace where these conclusions come from. The book is worth reading for this section.
Occasionally Harari gets facts wrong. You won't realize this unless you have investigated the matter separately. I noticed it because his description of the origin of the caste system in India was wrong, according to current research.
Harari tries to present himself as outside modern factions (or inter-subjective realities), such as nature vs. nurture, liberalism vs. conservatism, etc. But without conscious explication, he suffuses his book with the assumption that any modern human if taken from birth is equally at home in any of the current or historical inter-subjective realities. He does not propose or even consider experiments to determine culture-vs-genetics. So he proposes this important genetic ability evolved in a small population on a single continent between 130kya and 70kya, but that no differentiating evolution has occurred since then.
The question of whether the degree or style of inter-subjectivity is as universal as he implies is important for several reasons. Harari proposes the world is "different" since 1945, with no war between major powers, no more empires expanding by territorial acquisition. He suggests some reasons for this (cost of nuclear war, for example) which are unverified. His book was completed in 2014 before Russia claimed parts of Ukraine and China claimed the entire South China Sea. If inter-subjective capacity is universal, then this situation is likely unstable. People could quit believing it at any moment, and the world could return to any state that it has been in historically. If inter-subjective capacity is not identical in everyone, then it might make a great deal of difference which cultures dominate, even if through historical accident. See for example Boyd and Richerson 2009 Culture and the Evolution of Human Cooperation.
So, it is a book full of powerful ideas, often with carefully balanced arguments on both sides, but beware of accepting the background assumptions without critical thinking, or you will just fall into the latest meme.