The human brain is the final frontier. Scientists of all stripes have been examining it from every conceivable standpoint, tracking every electron it transports in any direction. We seem to know everything about it – how every little crease and fold defines an area with a specific job or jobs, what that area connects to, the pathways of the signaling – everything. Well, except for one thing: where is the human mind? In Joseph LeDoux’s latest effort, The Four Realms of Existence, he examines and explains the state of the art, and in particular, his own takes and the reasoning behind them. But in the end, it is all just description. The mystery remains.

While scientists have endlessly divided everything into twos (mind/body, physical/mental, body/soul, et al.), LeDoux has taken all his knowledge and decided on four realms that interact to make a human. They are biological, neurological, cognitive and conscious ways of being. Cleverly, he does not claim this to be a new scientific theory. That way he doesn’t have to prove he is right, face insane scrutiny or huffing and puffing challenges. Instead he calls these realms descriptive, and meant for descriptive use. And astoundingly descriptive they are.

Various other beings have stopped somewhere along the line, reaching the neurological but not the cognitive, for example. Primates are only ones with all four (to greater or lesser degrees). LeDoux does not speculate at what a fifth realm might look like if homo Sapiens can hang around long enough.

It is no exaggeration to say all scientific specialties are involved in this work. LeDoux himself cites “philosophers, psychologists, neuroscientists, biologists, mathematical modelers, physicists, and the press.” Since no one knows what it is they are looking for, the field is wide open. And remarkably, all of those specialties have and continue to contribute to our understanding. LeDoux has no issue citing any of them. Anything well thought out is welcome in science.

From reading this book, it would appear they have all gone about as far as they possibly can without hitting the jackpot. Yet there is plenty of room for positions, theories and arguments. The utterly vast storehouse of information on the brain remains totally insufficient to pinpoint what makes humans different from other mammals, other than listing things we do that they can’t. This is of course, not satisfying to anybody. LeDoux doesn’t even venture that we are closing in on it. He ends by saying he’s certainly along for the ride.



The book is overwhelming. There is so much information in it that it is simply not possible to retain it all, unless perhaps this is the kind of work you do all day for fifty years, as is the case for LeDoux. It threads its way through evolution, division of labor among the component parts of the brain and the current state of knowledge.

It is also remarkably thorough. Every time I came up with a yes but or a what about, LeDoux had anticipated me.

There are some areas where language itself keeps things unsatisfyingly vague. Sentience, for example. The definitions are so broad as for anyone to include pretty much any being beyond grass as sentient. And even grass has its moments. Scientists must work around these potholes in formulating their theories, and demonstrating their usefulness in seeking the holy grail. This is another reason why making the four realms merely descriptive is brilliant. It can be the basis for others to make leaps into the void, while LeDoux remains on solid ground.

Ledoux also takes issue with terms like personality and self. They are easily challenged, not very helpful, and remain stumbling blocks to understanding who we are. These two concepts are very recent and imprecise. The ancient Greeks, he points out, didn’t even have a word for self, while personality was relegated to masks in theatrical productions. Today, they are holding up progress in understanding the brain, the way LeDoux sees it.

He doesn’t deal well with memory, where it resides and how it is deployed, either. The best description I have seen is still this one from Patricia Churchland: Think of your first kiss. The memory is right there for you, even though you might not have thought about it in decades. We have no idea how that works. (In my review of Great Minds Don’t Think Alike ) ( https://medium.com/p/af3daa3edfd )

At one point, LeDoux weasels out of the mind issue by claiming it is a philosophical problem, not a scientific one. This would run into some rather hardcore scientific opposition, such as transfers, uploading to the singularity and maintenance after the death of the body. I was surprised to see it, but he insists that scientists must agree that consciousness is part of our physical, biological makeup and leave it at that: “Until we accept that (…) we’ll continue to be mystified by what it is,” he says.

But he doesn’t just leave it there. Fifty pages later he doubles down: “To scientists, non-physical consciousness is, or should be, an oxymoron.” For him, the wiring in the brain is all we are ever going to find. And no physical controller called the mind.

This is what it looks like as a system:

In the end, he comes back to psychology, and a tip of the hat: “Without the psychological descriptions that we humans extract from our folk-psychological understanding of our introspections, it would be impossible to know what brain circuits do. It literally takes a mind to know what a mind is.”

Onward, in circles.

David Wineberg