They saw the thunder
An array of noisy phenomena accompanied the Revelation on Mount Sinai. There was thunder. There was lightning (Ex. 20:15). But strangely, the thunder was seen and the lightning was heard, the opposite of what normally happens.
In the Mechilta, the rabbis draw our attention to what took place. They say, “They saw what is normally heard, and heard what is normally seen”. Both seeing and hearing are modes of sensory perception. The moment was so elevated that the experience transcended the usual way of the world.
If anyone had asked the Israelites to articulate what took place they would not have been able to give a straight answer. They would probably have said, “How did we know what occurred? We just knew!”
According to Ibn Ezra, their experience at the great moment was a miraculous combination of all their senses.
