The Book of Exodus relates that when G‑d gave us the Torah at Mount Sinai, "The people saw the voices." "They saw what is ordinarily heard," remark our sages, "and they heard what is ordinarily seen."

As physical beings, we "see" physical reality. On the other hand, G‑dliness and spirituality is only something that is "heard"--it can be discussed, perhaps even understood to some extent, but not experienced first hand.

But at the revelation at Sinai, we "saw what is ordinarily heard"--we experienced the Divine as an immediate, tangible reality. On the other hand, what is ordinarily "seen" —the material world—was something merely "heard," to be accepted or rejected at will.