Man is a being of volitional consciousness: beyond the level of percepts—a level inadequate to the cognitive requirements of his survival—man has to acquire knowledge by his own effort, which he may exercise or not, and by a process of reason, which he may apply correctly or not. Nature gives him no automatic guarantee of his mental efficacy; he is capable of error, of evasion, of psychological distortion. He needs a method of cognition, which he himself has to discover: he must discover how to use his rational faculty, how to validate his conclusions, how to distinguish truth from falsehood, how to set the criteria of what he may accept as knowledge. Two questions are involved in his every conclusion, conviction, decision, choice or claim: What do I know?—and: How do I know it? It is the task of epistemology to provide the answer to the “How?”—which then enables the special sciences to provide the answers to the “What?”Notice the relation between epistemology and the "special sciences": epistemology "enables" these sciences, so that it has, in effect, a logical priority and even a special authority over science. This is one of the most dubious and troublesome doctrines in the Objectivist philosophy. Rand believed that on certain points of doctrine, philosophy could dictate to science. The most egregious examples involve various metaphysical doctrines and their application by Objectivist "physicist" David Harriman.
But this is not the worst of it: there is an additional contention that epistemology provides science with a method of cognition, so that without epistemology, there can be no science. This is not true at all, and for a very simple reason: epistemology is not capable of providing a method of cognition, because no such method exists. There is no evidence suggesting that when people think, they follow a specific, formalized set of instructions. Again, as in many posts on this blog, I must invoke the cognitive unconscious, which, according to the latest research in cognitive science, does a great deal of the heavy lifting in cognition. "Mental efficacy" is not achieved via some formalized "method of cognition," but through the practical effects our thinking in everyday life. More often than not, the only test of mental efficacy is the degree of success or failure we achieve in everyday life. While there may exist a few epistemological precepts or strategies which may help us avoid error, none of these amount to a formalized method. Popper's philosophy of science is a strategy to help us avoid the pitfalls of confirmation bias and what Rand calls "psychological distortion." It is not a method of cognition, but merely a series of cognitive precepts that can improve our search for knowledge. The fact of the matter is that a certain degree of mental efficacy must already exist before the individual is capable of understanding Popper's philosophy of science or any other epistemological philosophy.
It is precisely because most of our thinking occurs below the surface of consciousness that epistemology can be so difficult to comprehend. Since we aren't capable of observing much that goes on as we think, we have difficulty relating to philosophical speculations concerning the machinery of the human understanding. When Rand arbitrarily declares that concept formation involves "a mental integration of two or more units possessing the same distinguishing characteristic(s), with their particular measurements omitted," it is difficult to evaluate that statement for the simple reason that nobody experiences his mind doing that. Concepts are formed below the threshold of consciousness, hidden from view. The Objectivist epistemology is difficult to understand not because it is particularly complex (it isn't), but because it has nothing to do with experience. As I stated above: it is largely speculative and imaginary.