Knowledge Without The Senses

To doubt if one's sensory perceptions reflect the external world's true nature is seen by a number of non-rationalists as if it is the same as denying that anything at all can be known.  For many, sensory information is the first thing that they might point to if asked for an example of something they know.  It takes only a slight push by oneself or others to be able to see, however, that the knowledge derived from the senses largely reduces down to mere perceptions of the external world, as the senses are incapable of demonstrating that the exact stimuli they perceive correspond to the true external world.

Rather than cast someone into an abyss of no knowledge about reality, though, understanding the epistemological limitations of the senses allows one to focus more on what can be known even apart from the senses: reason and oneself.  The senses, at an absolute minimum, do provide genuine knowledge of one's perceptions, but reason and one's own mind can be perceived as they are simply by reflecting on them.  There is no need to venture out into the world one perceives in order to discover purely logical truths or facts about one's own mental states.

Even if no matter of any kind existed, perceived nothingness would quickly yield recognition of logical truths, whether the perceiving mind was led to them through the acknowledgment of self-verifying axioms, the realization that it doesn't follow from one not perceiving sensory stimuli that nothing whatsoever other than one's mind exists, or the revelation that nothingness is itself something--which would mean that it could never be true that "nothing" is in existence.  The laws of logic that govern the nature of "nothingness" would always be separate from "nothingness" itself, and any truth that logic can reveal without sensory input would still be capable of being grasped.

In addition to knowing strictly logical truths, a mind that exists in a matterless reality would be able to identify and explore introspective truths about itself.  A mind could still have emotional reactions to its lack of sensory stimuli, for example, and it could also have certain attitudes towards logical truths (perhaps gratefulness for the absolute certainty they impart).  There is nothing about a lack of external stimuli that entails a lack of emotion, introspection, and inner life.  On the contrary, it is logically possible for these things to exist in the total absence of matter.

It is thus true that even the total nonexistence of matter and the senses would not bring a total lack of knowledge with it.  That which does not depend on matter and which does not need the senses is still accessible even if the external world were to vanish entirely.  As long as a being is capable of understanding logic, it is capable of knowing a plethora of metaphysical and epistemological facts pertaining to the nature of its own mind and of the laws of logic themselves.