Based on Michtav M’Eliyahu vol 1 p. 195-203
The source for man’s requirement to engage in hishtadlus is the cheit of Odom HaRishon with the eitz hadas. Man chose to draw himself closer to an experience of rah/concealment of G-d’s presence in order to increase the kiddush Hashem in choosing good despite the greater experience of rah. The consequence of this act was that he must now be committed to that process of always trying to see G-d in what looks like a natural universe; this is called hishtadlus. The amount of hishtadlus a person is required to engage in depends on how successfully it can facilitate his seeing G-d’s hand in the process. This depends upon where a person is holding in his belief about miracle and nature at the outset, as the following table illustrates.
NOTE: the word “miracle” can refer to either an aberration of nature or a show of hashgocha pratis