While we, today, no longer have that degree of direct knowledge, we do have the historical tradition of that experience, which (among other things) provides a rational basis for our belief in God and the Divine origin of the Torah. With that basis, we can then have faith, of the rational kind, that even when we do not understand why God does something, there must be a good reason for it.
It is only in this context that the concept of emuna peshuta - simple faith - comes into play. We trust God because we have simple faith in Him. But before we can trust him, we must first know that He exists. That initial knowledge cannot be based upon "simple faith."
It touches upon some of the same issue as I discuss here