An analogy can be taken from ice, as follows: If there is extreme cold which then turns to moderate cold, the rivers stay frozen and we can pass over them, for the ice is strong and thick. If, however, we see that the ice is not strong, this is an indication that the original cold spell was not that cold.

Similarly, if you see in yourself that sometimes you serve HaShemwith adhesion and sometimes you do not, know with certainty, that until this point, you have not served HaShemas fitting. For if you truly served Him as fitting, even once, you would always serve Him this way.