In my view, knowledge is a subjective feel, but there's a nice story behind it.
The subjective feel appears circular: I say that I know when I can't not know.
The story lies in the cause of this subjective feel: to know is to recognize.
Recognition is involuntary, hence the modality of 'can't not know'. It is also non-grounded. The subjective feel is the knowledge.
It's like recognizing a person. Your mother walks through the train station and you pick her out of the crowd. This recognition is not based on any particular rule or principle, not based on any essential features, not based on any inferential process.
Yes, you could be mistaken in the train station - an alien disguised as your mother may have appeared instead. But your knowledge is not at fault. You know what your mother looks like; that's where your certainty lies.
Recognition is a property of neural networks, and this is what explains why you recognize, why it's involuntary, and why it's not based on any rule or principle.
Your previous experiences seeing your mother have over time resulted in the growth of an associated pattern of connectivity in your neural network (ie., in your brain, ie., in you).
When a person with the appearance of your mother is presented to you this pattern is activated. It is automatic and involuntary. This pattern in turn activates associated patterns. Hence: mother!
The pattern is subsymbolic; the brain is not a physical symbol system. Hence, no rules or principles, nor either models or representations.
The neural network part of the story, note, is an *explanation* of how we know, and not the justification for it or evidence for that knowledge.