Reason is the inevitable result of intellectual survival
There has been a long-standing debate over whether mathematics was invented or discovered. One of the more solid arguments is presented by leading astrophysicist Mario Livio in his article "Why Math Works"
Interestingly, in that article, he refers to those who believe math was discovered as Platonists, due to the fact that Plato postulated the existence of a world of archetypes and laws that predated materialism. I would assume then, given Aristotle was not on board with Plato regarding this, that the "math as an invention" team could be considered Aristotelian... but I am not a philosopher, so don't hold me to that.
Livio gets into the idea that math is subject to the same evolutionary forces as species in that math that does not work quickly dies, never again to propagate itself into the mathematical 'gene pool.' Ideas such as the phlogiston theory of fire and Descartes theory of the motion of planets are examples of mathematical species that were quickly dispatched by the deadly and merciless hand of proof.
My son's small-town high school teacher answered this question surprisingly effectively: "Math is the language man invented to the describe the existing laws he discovered." It was a sufficiently satisfactory answer with regard to math, but I wondered if it applied equally to reason.
For years I had equated reason with math, as they both seemed to follow practical, testable, self-evident laws.
For example, the traditional "law of thought"  goes something like:
The Law of Non-Contradiction states "Nothing can both exist and not exist at the same time and in the same respect, or no statement is both true and false."
The Law of Excluded Middle states "Something either exists or does not exist, or every statement is either true or false."
These are laws are much like math, in fact, they have their own algebraic notation: ∼(p · ∼p), p ∨ ∼p, and F(x) ⊃ F(x) or (∀x) (x = x), respectfully.
It seems reasonable to assert that these laws existed before we came up with a way to define them. We certainly did not invent the fact that something can "be" and "not be" at the same time. But is it true?
When I first heard these laws my reactions was "It depends on the thing in question, and what you mean by the word 'exist'".
A hardcore Aristotelian materialist might say "The law of non-contradiction specifically forbids something existing and non-existing. The universe has a transcendent self-consistent order, and that order cannot be violated. Its primary manifestation is thus: it exists, and it cannot be otherwise" while a Hindu guru might say " God is both Sat (existing) and Asat (non-existing)." It would appear that the laws of reason depend on what one believes to be transcendentally true to begin with.
The Aristotelian is easily able to define, and defend, his worldview, while the guru is easily able to do the same. They are both able to recognize the reality they have accepted as true, supported by the reasoning of that truth.
Before one can proclaim some thing's existence, that thing has to be known. We can't say something exists if we have no idea what it is we are making that claim about. I can say my dog exists because I know of dogs, and of my own dog, but can I say Phlimquitz exist, and when you ask me what a Phlimquitz is, I say "I have no idea"? That would be crazy (i.e. extremely unreasonable). But if I said Phlimquitz explains the 99.97% correlation between "U.S. Spending on science, space and technology" vs "Suicide by hanging, strangulation and suffocation", then I see recognize something others don't and could go about testing my hypothesis.
BTW, here is that correlation, just for fun:
The same goes for God, angels, spirits of the dead, the Djinn, magic forces, kundalini, chi, mana, etc., etc.. Many people have a clear understanding of such a concept, and as such, they are able to say it exists because they can recognize it. This is even more explicit with the research of OOBEs (out of body experiences) . Almost every culture in the world has a concept of some sort of paranormal, metaphysical energy, and some cultures have extremely demanding and lengthy training to tap into it. Even our western icons, such as Paracelsus, Hippocrates, Carl Jung, Erwin Schrodinger, and Plato have referred to it.
So, what then is the ultimate arbiter of proof as to whether something is "reasonable" or not? It would seem like there is no fool-proof way, no absolute proof, that any one way of looking at something is the "right" way. However, we do have a gauge by which to measure if something is sustainable, which may be the best test as to the validity of something, and that gauge is life itself.
The claim here is that the nature of life is the most basic order by which the existence of anything must adhere to in order to continue being.
Let's take a look at a single cell, the building block of life. This is high-school biology, but let's go over it again.
Inside the cell are two meters of tightly wound strands of four very specific molecules, held together by sugar phosphates, that make up the DNA of that cell. These are C5H5N5 (adenine), C5H5N5O (guanine), C4H5N3O (cytosine) and C5H6N2O2 (thymine). They are arranged in a very specific order which describes the instructions necessary to build every protein in an organism.
In a process known as transcription, a molecular machine first unwinds a section of the DNA helix to expose the genetic instructions needed to assemble a specific protein molecule.
Another machine then copies these instructions to form a molecule known as messenger RNA.
When transcription is complete the RNA strand carries the genetic information through the nuclear pore complex, the gatekeeper for traffic in and out of the cell nucleus.
The messenger RNA strand is directed to a two-part molecular factory called a ribosome.
After attaching itself securely the process of translation begins.
Inside the ribosome, a molecular assembly line builds a specifically sequenced chain of amino acids.
These amino acids are transported from other parts of the cell
They are linked into chains often hundreds of units long. Their sequential arrangement determines the type of protein manufactured.
When the chain is finished it is moved from the ribosome to a barrel-shaped machine.
It is then folded into the precise shape critical to its function.
This process, and so many others, are so specific, so complex, so difficult to explain with current evolutionary models, that Dean Kenyon, a Professor Emeritus of Biology at San Francisco State University who actually taught Evolutionary Theory, switched teams in 1979 and went on to be a key member in the "Intelligent design" movement.
I am not going to argue the merits of Intelligent Design vs. Evolution here. Not because I believe in one over the other, but because I believe they are both true.
The important point for this post is that if any one thing in the list of processes listed above changes, if one atom in the C5H5N5 molecule was different, the cell would either die or mutate. This is actually what cancer is, the malformed reproductions of cells.
Only one process, made up of numerous details steps, all of which much functions properly, will result in successful reproduction, and more importantly, we can explain why one, and only one out of the countless theoretical options at each step of the process will work.
Evolutionists will say that all the other options, at one time in our past, were tried, and failed, leaving only the process that worked. Intelligent Designers will say that this was "biochemical predeterminism", i.e. that the process was preprogrammed, somewhere... perhaps in the "morphic field" that Sheldrake theorizes . I certainly don't know, but I don't need to know to say that whatever the reason, one process that follows a strict set of rules and conditions must exist.
These rules and conditions have been following a very perfectly reasonable path long before man was able to claim them as such. In fact, the very proof of that "reason" is that it follows such a path. We know that DNA is a helix because of hydrogen bonds between the phosphates. We know that the nucleotides are covalently connected to the phosphates and that the nucleotide pairs are connected with a weak hydrogen bond, making them easily disconnected. Because we know this, because we discovered this, we can say "Here is a rule", and anything that follows that rule is "reasonable", because we have the evidence of its "reason" right in front of us in every living cell. This is also why we say that raping a virgin will not cure one of AIDS, a practice happening right now in South Africa, is "unreasonable", because there is nothing in nature that would support this rationale.
Creation myths of the cosmos are unreasonable because the hows and whys do not conform to natural law, while astrophysics is reasonable because we can explain the hows and whys according to natural law. But saying "The universe has a transcendent self-consistent order" also seems to be bordering on unreasonable, as we know so little about the universe, I mean, we only discovered that there were other galaxies in 1929. It would a bit arrogant to start claiming what the transcendent laws of the universe are at this point.
This is whay I think Reason is the codification of the rules of reality as we have come to know reality.
This is not to say that all myths, magic, aliens, hyperdimensional beings, and numerous other paranormal, metaphysical areas are defacto "unreasonable". It was not that long ago that electricity, flight, wireless communications, etc., were nothing less than magic until we were able to recognize it and discover the rules of how they worked. Once we discovered Bernoulli's law takes precedence over the law of gravity we learned to fly.
If I think of Intelligent Design as a fractal-like pattern of moving energy that extends far above and below our sphere of reality, that sliver of the reality spectrum that we are attuned to perceive, then ID is perfectly reasonable, because it would imply that we exist in a pre-patterned holofractal-ish reality. We assign ID to God now because we haven't yet learned the rules or how it works, but we have to imagine someone, or something, that does. With ideas such as the wave functions and the many-worlds theory of quantum physics, the morphic field of biology, the research into hyper-time telepathic communication (for long distance space travel), it's easy to see how what we accept as "reasonable" will dramatically change as we learn how these new processes of reality work.
In short: The rules of reason are predetermined based on the rules that this reality requires anything that exists to abide by, because it is these rules that provide the proofs of reason.
However, the true prison of the mind is not these hard and fast empirical rules, but the dogma that surrounds them in modern thinking. For example, astrology has plenty of reasonable support in the form of scientific research. Coincidentally, as I was researching this post I ran across the article History shows that astrology was discovered, not invented.
There was quite a bit of early scientific research into vitalism before the materialism thing took over the sciences in the early 19th century, but the whole "Mind-Body" approach to health sciences is something like a rebirth of vitalism.
Most astounding is the research done on ESP which resulted is statistical evidence that far outperformed the research that showed aspirin helps reduce the chance of a stroke - a study that was so convincing it was considered unethical to even continue giving placebos to the test subjects. Here is Russel Targ speaking about this research.
There is so much research that could shed light on a new paradigm of reality that is either being ignored or outright blocked so much so that it will be decades before we begin to escape from the dogma of materialism that holds over our lives, including our ability to reason in a way that is actually harmonious with the reality we actually live in, but know so little about.