When ever there is doubt, the practitioner must ask, "who is it that doubts?" When this source is uncovered what is there left to doubt?
Knowing what he knows and knowing what he doesn't know: finding his way back home.
Dhamma concepts must be clearly understood at the intellectual level before they can be intuited symbolically. Only then can they be operated on with symbolic logic: that is to say Cause and Effect, Dependent Origination, Here and Now, Conditionality, etc.
Being Dhamma, this won't include Space-Time systems unless the practitioner wishes to look at Dhukka, thus becoming mindfully careful with this sticky Kamma.