The first malleable faculty I considered was memory as it pertains to mnemonics. The ancient greeks considered a mnemonic education essential to any citizen's intellectual development. They knew that one's intellectual capacities to remember were vastly plastic and required constant discipline to maintain high performance. There is no absolute structure that underpins everyone's memory the way there is for early vision. Some people think in pictures, others in words and some kinaesthetically*. But, more importantly, an individual can alter the architecture of their memory, either by choice or through externally generated environmental cues. Our ability to influence memory begins with attention; the way we see the world during perception. We can chose to pay attention to particular features of our world which will in turn affect what we retain at a later time. We can construct mnemonic schemes to sort incoming information into more easily retrievable chunks. If we have poor spatial or imagistic recall, we can practise to improve these areas. Like the muscles of the body, our memory is incredibly malleable. Now, of course, we will not all have infinite capacities for change. Just as men tend to have an easier time building muscle mass than women, some individuals will be innately disposed for greater mnemic potential. However, within certain biological constraints there is a vast area for work and improvement.
Another malleable faculty is introspection. Many ancient cultures have argued that through meditation we can come to know our thoughts mental states and feelings with greater clarity. The details of this process I will have to expand on at a later time.
Malleable faculties are particularly interesting from an epistemic point of view because they reveal that we can become more reliable, more knowledgeable through greater technique. Unlike static epistemic issues in perception such as blindspots, change-blindness or visual illusions, we have more control over the errors of our memories and introspection. With mental exercise we can navigate the world less prone to error. Of course, this does not mean we can avoid being tricked altogether. Memory is an inherently falliable faculty, so no amount of training can entirely eradicate error.
If memory and introspection are malleable, what sort of faculties are not open to conscious will? I argue that all the cognitively impenetrable candidates are likely immutable, e.g. early visual processing. But, perhaps also our faculty of desire and our faculty of belief. For, although we can alter the content of what we believe and what we desire, we cannot change the architecture that manipulate these propositions. Perhaps this is unfair on belief? Perhaps we can become more rational and more logical and thus more reliable at forming true beliefs or maintaining skepticism when appropriate? But, surely this is not the faculty of belief, but the faculty of reason, of thought itself? Reason may be another malleable faculty.
* We should differentiate between the way we introspect our mental states from the structure of representations underlying the phenomenology. e.g., when imagining Paris we might say, "I see the Eiffel Tower", yet have no mental imagery whatsoever. Our use of metaphor in speech (e.g. I see what you mean) is so pervasive and subtle that we must be careful before attributing significance to it. Nevertheless, there do seem to be different ways of experiencing mental states and they can involve physical differences such as emotions or bodily sensations, auditory or visual experiences.