The Mental Plane: Other Minds

The most genuine form of privacy possible goes far beyond enjoying untracked Internet browsing or having one's body go physically unseen by others behind a material barrier like a wall.  It is only brought about by the inability for other beings to see within one's very mind.  You could be right in front of them, speaking or staring, and they could never know what you are thinking, whatever accurate or misleading clues are evident in your expressions, gestures, or other behaviors.  This utmost privacy appears to be taken for granted by many people, though they might painfully fixate on it in some situations.

Even then, they might overlook the fact that the mental plane is only seemingly shielded from other human minds, for if one cannot see into other minds (if they truly exist), one has no way of knowing if they can see into one's own.  It is just that it really does seem that they do not, as evidenced by the reactions people make when you do something they would probably have thought unexpected.  For instance, something as simple as a surprise birthday celebration would not be possible if people truly knew what others are thinking or that other minds are metaphysically present even if the presence of their physical bodies is concealed from the senses.  No one could ever be surprised in the fullest sense by other people at all if other minds were known.

Likewise, if someone acts shocked at evidence surfacing that another person's earlier statement is a lie, they probably, as far as the fallible, perception-based evidence suggests, could not see into the liar's mind at an earlier time if they did speak a mistruth.  A lie only becomes a deception if the party lied to believes the falsity, and no one could be deceived if people truly saw each others' minds instead of merely observing their behaviors or hearing their words.  Deception is logically impossible unless minds really are not epistemologically (and thus to some extent metaphysically) connected.  While it is not the same as lying, anyone who has ever struggled to keep from divulging information to another person is directly or indirectly hoping that the other party cannot simply gaze into their thoughts.

One's own inability to see into other minds does not mean any other minds that exist would have the same limitation, though: that is, there is no way to know from one's own epistemological boundaries that other beings would share the same confinement.  With non-necessary truths, what is true of one thing does not have to be true of another.  The latter in this case does not logically follow from the former.  This does not exclude some separate being like God gazing into one's thoughts, or even other people as far as I can prove/know, but the mental plane of each conscious being is still directly accessible to itself either way.  What is immediately experienced cannot be an illusion for someone making no assumptions--without avoiding assumptions, though consciousness along with logical axioms cannot be an illusion, a non-rationalist still cannot know this.  They can only assume.

One's own mind is indeed all that can be known other than the inherent truths of reason, such as logical axioms, and miscellaneous other facts entailed by them.  One cannot know from the sensory perception of vegetation that the plants really are there outside of mental experience, but the perception exists either way.  One cannot know from the fact that it appears like other human minds exists that they do, but one's consciousness is there to experience the fallible evidence regardless.  The mind is knowable through rationalistic introspection with absolute certainty.  Whether the real or seeming privacy it affords from other beings is treasured as a blessed relief or lamented as a source of unwanted isolation depends entirely on a person's subjective dispositions.