This is an excerpt from the book: The Knowing Universe.

A philosophical and scientific theory with the potential to explain existence appears to be coalescing around the variational free-energy principle (FEP). Almost two decades ago, Karl Friston introduced the FEP as a unified neuroscientific theory. These initial papers are now some of the most cited in the field and have inspired thousands of other papers developing his principle and applying it to everything. Great excitement is building from a growing body of evidence hinting that the same general strategy brains use to keep their hosts in existence may be common to all forms of existence.

While this approach has gained tremendous popularity among
researchers, it has also baffled many experts. As Wikipedia tells us

*The free energy principle has been
criticized for being very difficult to understand, even for experts.*

Although this simple-sounding principle is transforming
cognitive neuroscience and is considered by many (myself included) as the most promising
approach to a theory of everything, the bafflement it induces in smart people
is legendary.

In contrast to its supposed difficulty, I marvel that it
makes such clear sense. How could I so easily comprehend this principle, which
seems to escape much brighter people? Perhaps the answer is that my unusual
intellectual journey has arrived independently at many of the conclusions underlying
this principle, and without these distinctive insights, it may well have remain
incomprehensible to me. The upside is that perhaps this background may offer
some assistance to those struggling to understand the FEP.

We can easily state the principle: everything attempts to
minimize the surprise it experiences. It sounds pretty innocuous for a theory
of everything; in fact, it has an almost Zenlike simplicity. But like a Zen
koan, its meaning is elusive. Many papers fail to fully explain the principle
before diving into complex mathematics and computer simulations, and some readers are
left wondering about the claimed links between surprise and existence.

The ability of all things to experience surprise contains
one critical assumption that is rarely explicitly mentioned. The assumption is
this: the existence of every ‘thing’ depends on a model having knowledge for
its self-creation and maintenance. This model provides an expected roadmap for
existence, and surprise occurs when its expectations are unmet. For many of us,
the idea of everything having built-in models that can be surprised is a little
hard to accept. But consider that all life has genetic models and complex
animals have neural models, and humans have cultural models. Each of these
models can be surprised by the evidence. Friston sometimes uses the example of
a fish whose genetic and neural models expect it to be in the water and are
surprised if it is not. Surprised genetic and neural models are often
precursors of death, and surprised cultural models are often precursors of
cultural extinction. That is why all things attempt to avoid surprising their
models; things that don't trend towards non-existence.

What about physical existence? As we discuss in chapter 8,
it turns out that the quantum wave function may form a similar model for quantum
existence, but the argument is somewhat more complicated

This connection between models and existence is profound and
deserves some explanation. Why should existence require a model? The short
answer is that the challenges to existence are formidable, and existence does
not occur without following a detailed, knowledgeable model. But we see
existence all around us. In what sense is it challenging to achieve? A law of
nature, the second law of thermodynamics, summarizes the challenges to
existence: disorder increases in all things. If a thing's disorder increases
enough, it ceases to be that thing; it becomes non-existent. As we see a little
later, the second law and the free energy principle say much the same thing,
but while the second law focuses on existence's challenges, the free energy
principle focuses on their circumvention through reducing their models' surprise.

But how do things act to minimize surprising evidence? There
are two answers: things can accurately follow their models and produce evidence
confirming their model predictions, or secondly, they can improve their models to
make better predictions. In short entities can either cause reality to conform to their model or cause their model to better conform to reality. The first strategy is easy to comprehend as our genetic, neural,
and cultural models predict existence enhancing outcomes and, as a bonus, provide
algorithms for achieving those outcomes. Thus this route to minimal surprise only
involves following the models as accurately as possible - anything's best
strategy for existence is to reduce errors in executing their finely-honed models.
The second answer is the evolutionary processes that create and hones more
knowledgeable models. This process called inference uses a thing's relative ability
to achieve existence as evidence and uses this evidence of existence to update their
models' accuracy; think natural selection where evidence generated by the
struggle for existence updates the genetic model — the more knowledgeable the
model, the fewer surprises it experiences in the world.

This principle's beauty is in its mathematical depth;
Friston and colleagues have developed mathematics to approximate surprise
experienced in complex, real-world phenomena.
Here we only scratch the mathematical surface to reveal a bit of its
potential.

We should probably start with the mathematical definition of
surprise; it is -ln(p), where p is a probability that some hypothesis is true.
How does evidence create this surprise? When sufficient evidence reveals the
truth of a particular hypothesis, then -ln(p) is the surprise experienced; if
the initial probability assigned to the hypothesis is small but the evidence
indicates that the hypothesis is true, there is much surprise.

What does -ln(p) have to do with an entity's model? Models
used by real-world things to achieve their existence are probabilistic models. Genetic,
neural and cultural models involve a family of competing hypotheses, each of
which is assigned a probability that they are the one true hypotheses. For
example, at each of an organism's genetic locations or locus, various
individuals from the population may have different genetic sequences or
alleles. The probability assigned to each specific sequence is its relative
frequency within the population, and this probability is the fitness of the
sequence. If over many generations a
population evolves from having multiple alleles at a locus to having only one,
the probability for that sequence is 1, and we might say that the evidence has
proven it to be the fittest among the initial family of alleles; it is the one
proven to produce the least surprise among the options.

We can consider the hypothesis assigned probability p as one
in a mutually exclusive and exhaustive family of hypotheses offering solutions
to a real-world existential challenge. Being mutually exclusive and exhaustive
has a couple of consequences. The first is that one and only one of the
hypotheses must be true. The second is that the sum of the probabilities over
the family of hypotheses must equal 1. If the probabilities add to less than 1,
then the hypotheses are not exhaustive; some other possibility exists. If the
probabilities add to more than 1, they are not mutually exclusive; the
hypotheses have some logical overlap.

Real-world instances simplify these mathematical
complexities. For example, the family of alleles at a genetic locus within a
population of organisms is naturally mutually exclusive and exhaustive. It is
mutually exclusive because each allele is unique, and it is exhaustive because
the family consists of all the alleles within the population. Thus the sum of
the relative frequencies of alleles in the population must equal 1 as that is implicit
in the meaning of relative frequency.

Because the probabilities assigned to the family of
hypotheses sum to 1, they form a probability distribution, and a good deal of
mathematical machinery is available for analyzing probability distributions.
For example, every probability distribution has the property of entropy or the
amount of expected surprise: Sum(-p ln(p)). Thus minimizing free
energy is equivalent to minimizing model entropy. But the second law of
thermodynamics states that the entropy of isolated systems must always increase.

It is in this seeming contradiction that it all comes
together. Systems having unconstrained entropy are subject to unconstrained
surprise and dissipate into non-existence. An alternative statement of the free
energy principle is that existence depends on minimal surprise. Systems only achieve existence if they know how
to avoid isolation and exploit outside energy sources to decrease their
entropy. And they must accomplish this while following the second law in producing
entropy increases in the combined system plus environment. For example, a
photosynthetic cell's existence depends on its genetic knowledge for using the sun's
energy to counter the second law's tendency towards disintegration; the combined
cell-plus-sun system's entropy increases as dictated by the second law and more
than pays for the cell's entropy reduction.

Existing systems follow their models' knowledge to navigate
the environment and fend off nature's relentless forces towards dissipation. In
short, existence is fiendishly tricky; it requires a great deal of knowledge to
achieve and must follow that knowledge without errors or surprises. The
free-energy principle is important because it is a road map, perhaps nature's
only roadmap, for achieving existence, and that is why it provides a principled
account of all things.

# References

**Friston Karl** A free energy
principle for a particular physics [Journal]. - [s.l.] :
arXiv:1906.10184 [q-bio.NC], 2019.

**Raviv Shaun** The Genius Neuroscientist Who Might Hold the Key to
True AI [Online] // Wired. - Wired Magazine, November 13,
2018. -
https://www.wired.com/story/karl-friston-free-energy-principle-artificial-intelligence/.

**Wikipedia** Free energy principle [Online] // Wikipedia. -
3 11, 2019. - https://en.wikipedia.org/wiki/Free_energy_principle.