[RightWhale]

there are sources of subjective knowledge that are not tied exclusively to sense perceptions

Space and time and mathematics such as geometry and algebra for starters. Also the categories, be they Aristotle's or Kant's or somebody later who wonders where these particular categories came from if not from experience. All these are a priori or concepts not from sensation.

[betty boop]

...somebody later who wonders where these particular categories came from if not from experience.

If we equate "experience" with sensory contact with the reality exterior to ourselves, and to nothing more, then it's perfectly understandable that a person would have to "wonder." It seems to me there's nothing in this sort of experience that can, all by itself, account for them.