Well an example would be if the aircraft is sinking but the pilot is experiencing the sensations of gaining altitude because he feels the lift in the seat, he s

Well an example would be if the aircraft is sinking but the pilot is experiencing the sensations of gaining altitude because he feels the lift in the seat, he sees the angle of the wing against the horizon, and he hears the aircraft making the distinct noise it makes while climbing. But the instruments may tell him that he is not in fact climbing from the earth, he is descending, and he just feels like he's climbing because in the column of air he dwells, he is climbing. But that column of air itself is falling. This disconnect between senses and reality is the whole reason even pilots that only fly visually require instruments. On a horizontal plane it's much easier for a person to determine if they're off course using visual reference points. For altitude it's much harder to determine using just your senses.