But subjectivity is certain.
Lee: Since the only thing that is certain is "I think therefore I am"
"...I am thinking", it's not a stretch to say that no worthwhile
knowledge is certain. All knowledge is conjectural. To be fair,
you should google for "Pan Critical Rationalism" if you have not
already read up on it.
Only scientific knowledge is conjectural.
Only third person communicable knowledge is conjectural.
You did acknowledge the ineffable knowledge of what is is like having a
friend putting a needle in your finger.
And, of course, I would not be happy if when I complain about headache
to my Doctor if he tells me "your headache? Pure conjecture!"
First person knowledge is not conjectural, at least not consciously so,
nor consistently so.
It is Descartes fixed point of its systematic doubting procedure, when
you doubt that you doubt making up an unavoidable place for an
indubitable reality, though ineffable.