What the Bible says about Ability to Perceive
(From Forerunner Commentary)
A spirit being is invisible just like wind or air is to the unaided eye. The wind and the air are real, just as spirit beings are real, and they have substance just as wind does. But the unaided eye is not able to spot them, so the ability to see something that is spirit, or composed of spirit, is not in us by nature. Though it is not there naturally, the ability to perceive them can be given.
This is what Elisha means: "Give him the ability to see what is around us." And so the young man apparently saw a tremendous army of spirit beings who were ready to do battle in Elisha's behalf should anything occur. They were there all the while, invisible but nonetheless there.
John W. Ritenbaugh
Image and Likeness of God (Part Four)