In Anon's most recent reply, he focuses largely on the issue of explaining why "the concepts we need to learn first are just the ones whose referents have the most obvious perceptual similarities." I'll leave it to cognitive scientists (including those mentioned by Anon, along with others) to determine the degree to which this part of the theory accords with reality. In the meantime, I want to focus at what I consider the chief point at issue: the hierarchy as a theory of justification. As Anon admits, the theory attempts "to show how abstract reasoning connects to perceptual evidence." Does it really? But how can that possibly be? At the very most, it only shows how the concepts in use among propositions relate to perceptual evidence. However, abstract reasonings are used to formulate theories; and under no circumstances is it plausible to conclude that a theory can be justified merely by showing "how one's higher-level concepts (and associated propositions) derive from lower-level concepts." Knowledge about very complex aspects of reality — knowledge of economics or politics, for instance — is expressed in theories, and theories are more than the sum of their concepts or the logic of their propositions. It is entirely possible that a theory could be made up of valid concepts and pristinely logical reasoning yet still fail to accord with important facts. The reason for this is quite simple: social theories are (and must be because of the complexity of the subject matter) based on simplified premises. Economics, for instance, seeks to understand what would happen under simplified conditions never realized, but often closely approached in practice. Knowledge of reality involves achieving human scale, and that will sometimes involve a certain degree, hopefully within tolerable limits, of cognitive inadequacy. Sometimes the best we can hope for in a given branch of knowledge is merely to achieve a tolerable degree of plausibility.
Rand does not appear to have accepted the notion that theoretical knowledge of subjects like economics and politics might be characterized by a unspecifiable degree of uncertainty. She presented her own notions on these matters as if they were quite obviously the certain conclusions of reason, which any rational and moral person would accept as a matter of course. And she appears to have believed that her validation of concepts, of which the hierarchy theory (along with measurement omission) is a critical part, removed any problems related to the possible distorting effects of condensation. Nor, sad to say, does she appear to have confronted the problems of the actual simplification that takes place in the process of devising the premises of theories. The hierarchy of knowledge does not, and cannot, address this issue, since the problem has nothing to do with whether the concepts (or even the propositions) are "connected" to reality, since the way in which any really sophisticated theory "connects" to reality is far too complicated to be tested by tracing its concepts back to their perceptual concretes. Theories "model," in greatly compressed form, certain aspects of reality. When we talk about theories or concepts or propositions being "connected" to reality, we are speaking metaphorically, not precisely or literally. Theories don't mirror reality; they reduce an immense complexity to human scale, so that the mind can get some inkling of that complexity. Theories of complex phenomena are not proved or corroborated or justified by tracing their concepts back through a hierarchy to their "perceptual base." Perhaps that would be possible with something really simple, like a theory of furniture. But it's not in the least possible if we are talking about the theory of the fall of the Roman Empire, or an economic theory of tariffs, or a theory of the circulation of elites within a socio-political system.
So how do we know if these theories are true if you can't connect them back through the "hierarchy" to the perceptual base? Science uses rigorous empirical testing to corroborate the conclusions of a theory. No need to trace anything back to the perceptual base from which reasoning proceeds; no, simply test the conclusions! Much simpler, and far more convincing. But there are situations when, because of the inability to run experiments and isolate variables, such empirical tests can hardly be considered conclusive. In economics, for instance, one can't provide entirely convincing empirical tests "proving" that, under all conditions, tariffs will reduce the total output of an economy. The most you can hope for in fields of study where empirical tests yield ambiguous results is to attempt to determine, on the basis of abstract reasoning from simplified premises, what is likely to be true or plausible. Certainty (or near certainty), whether of a contextual variety or otherwise, does not appear possible in such disciplines.
Now, the belief in an epistemological theory that promises what it can't deliver will likely lead to cognitive mischief. The danger is that the theory will give its proponents an unjustified confidence in "abstract reasoning." Even if the intention of the hierarchy is merely to justify so-called "justified" speculation, if the theory does not deliver on its justificationist promises, it will likely lead its proponents unwittingly down the primrose path of rationalist speculation. Isn't this what we find in Objectivist thought — unjustified speculation run rampant, yet all done with the most naive sort of cognitive innocence? Rand herself made any number of assertions about matters of fact that are grossly implausible, and yet she seems to have made them with an entirely serene and untroubled epistemological conscience. For example: "Man is born with an emotional mechanism, just as he is born with a cognitive mechanism; but, at birth, both are 'tabula rasa.'" "Emotions are the automatic results of man’s value judgments integrated by the subconscious." "Emotions are not tools of cognition." “[A man’s] body will always follow the ultimate logic of his deepest convictions.” "A man is a ... being of self-made soul." These statements are not presented as possible conjectures or yet-to-be-proved hypotheses, but as veritable certainties. Some of the statements are very likely false. You would be hard pressed to find any scientist acquainted with the relevant evidence who believes in the blank slate model of human nature advocated by Rand. Nor are Rand's views of emotions any better supported by scientific evidence, as Damasio, among others, has discovered. Her statements relating to human nature are, in the light of evidence compiled in genetics and cognitive science, immensely implausible. Her justificationist epistemological theories have here let her down! Instead of leading her to truth and enlightenment, they have mislead her into error and delusion!
I don't see among Rand or her followers any appreciation for how difficult it is to determine the veracity of certain types of truth claims. Too much confidence is placed in reasonings based on vague generalities or insufficiently detailed understanding of the subject at issue. The consequence is a philosophy characterized by an overabundance of implausible and semi-plausible assertions. My suspicion is that Rand's justificationist theories, including her hierarchy theory, by giving her an unwarranted confidence in her own reasoning abilities, served merely to fortify her rationalistic and dogmatic tendencies.