In building a model of the grammar of a natural language, we often find that (1) two different linguistic forms have the same logical substance and (2) the same linguistic form has two different logical substances. For example: (1) The two words “have” and “has,” as in “I have a cold” and “he has the flu,” differ only in linguistic form; their logical substance is the same. (2) And the one word “have,” as in “I have a cold” and “he could have gotten the flu” differ not in linguistic form but in logical substance. The former meaning of “have” is obvious. If you “have” a cold, then you’re in that state. The latter meaning, by contrast, isn’t so obvious. It’s that of the past tense. Compare “could” and “could have,” “would” and “would have.”
Imagine building a system of logical notation that has a way of symbolically representing the past tense, the present tense, and the future tense, along with many of the other logical distinctions underlying the grammatical system of natural language. In effect, you’d be building a differently aligned language, in its connections between the surface form of its symbols and the deep substance of its meaning. You’d be realigning, reclassifying; you’d be building a different order.
What we do in physics is analogous. In doing physics, or (in other words) in building a physics model of sensation, we often find that (1) two different sensory forms are caused by the same physical substance and (2) two different physical substances cause the same sensory form. Physics, then, in an analogous way to logic, is an artificial realignment for a philosophical or scientific purpose. Physics replaces the “natural language of sensation” (metaphorically speaking) with an “artificial language of sensation.” In that way: Linguistics is to logic as the introspective psychology of sensation is to physics. Just as linguistics and logic study two separate but interconnected orders, one being more natural than the other, the introspective psychology of sensation studies the “sensory order,” as Hayek calls it, and physics studies the “physical order.”
The natural science of the psychology of sensation, then, studies the “neural order,” which is the interconnection between the sensory order and the physical order. According to Hayek, something in (the physical structure of) the brain, along with the rest of the nervous system, must be isomorphic to (the mental structure of) of the mind. What that “something” is, which can be theorized right from the start of the inquiry as necessarily existing, is left as an open question for me (and not a very highly prioritized one)—though Hayek does make some interesting guesses—for the philosophical significance of Hayek’s starting point in theoretical psychology to the question of the epistemology of the sciences of human action and the human mind doesn’t depend on the answer to that question, i.e. the question of what that “something” is.
The significance is simply that building a properly founded natural science of human behavior and the human nervous system, without sneaking in through the back door anything that’s just directly taken from introspective psychology, would be extraordinarily roundabout (not to mention of dubious utility to economics, linguistics, and the other praxeological and thymological sciences).
That is: We’d have to go from the sensory order, as we experience it naturally, to the physical order, and then we’d have to turn back, going from the physical order, through the neural order, all the way back to the sensory order, but no longer as we experience it naturally: We’d have to build a sufficiently accurate and detailed (physical) model of the brain, along with the rest of the nervous system, that’s in some way isomorphic to a correspondingly sufficiently accurate and detailed (mental) model of the mind. And then we’d just be right back where we started!