One can never be sure if one has them (6)
I believe the answer is:
doubts
If this is the answer, I'm afraid I cannot explain it.
Can you help me to learn more?
(Other definitions for doubts that I've seen before include "Thinks something unlikely" , "Isn't certain about, has reservations" , "Has reservations, is uncertain" , "Suspicions" , "Misgivings" .)