In Parshat Vaetchanan, the Torah alludes to an important facet of the events at Har Sinai.

רַק הִשָּׁמֶר לְךָ וּשְׁמֹר נַפְשְׁךָ מְאֹד פֶּן־תִּשְׁכַּח אֶת־הַדְּבָרִים אֲשֶׁר־רָאוּ עֵינֶיךָ וּפֶן־יָסוּרוּ מִלְּבָבְךָ כֹּל יְמֵי חַיֶּיךָ וְהוֹדַעְתָּם לְבָנֶיךָ וְלִבְנֵי בָנֶיךָ

But take utmost care and watch yourselves scrupulously, so that you do not forget the things that you saw with your own eyes and so that they do not fade from your mind as long as you live. And make them known to your children and to your children’s children: (Devarim 4:9)

This Pasuk commands us to remember the events of Har Sinai. But the focus of the Pasuk seems to be misplaced. The most important aspect of Har Sinai would seem to be the Aseret Hadibrot, which we heard from Hashem. If so, why does the Torah command us to remember the things that we saw, as opposed to the words that we heard? Granted, there was a spectacular display of fire and smoke, but those were secondary in importance to the actual giving of the Torah.

In Parshat Yitro, the passuk says

וְכָל־הָעָם רֹאִים אֶת־הַקּוֹלֹת וְאֶת־הַלַּפִּידִם

And the entire nation saw the voices and the flames (Shemot 20:15)

The Mechilta writes that the Jews were able to see that which was normally heard, and hear what was normally seen. What does this Midrash mean?

The Nefesh HaChaim writes that God gave man senses in order to take in information from his surroundings. The most reliable and trustworthy sense is man's vision. We find this idea in Masechet Rosh HaShana, where the gemara says that if Beit Din were to see something, that is even better than hearing it.

But Jews believe that just because something can't be seen, doesn’t mean that it doesn’t exist. Hashem says that no man can see Him and live, yet our entire worldview is built on our belief in Him.

Maybe, at Har Sinai, the Jews were able to 'see' and verify the belief in Hashem, which normally can only be 'heard'. The Jews heard the Aseret HaDibrot, but they had such amazing clarity at that moment, that they believed beyond any doubt that God and His Torah were true. From that moment on, the Mesorah was passed down through word of mouth. Therefore it is critical to remember that our tradition goes back to the time where we saw the truth.

The Midrash also said that the Jews heard what was normally seen. What does this mean? Maybe the explanation is that humans are tempted to believe that what we see is what there is. We see a magnificent world with so much intricacy and beauty. But at Har Sinai this vision was downgraded. We realized that the entire cosmos can't exist for even one moment without Hashem's Will. What seems like the most concrete reality is completely subjugated to Hashem.

The Gemara in Bava Batra 75 a quotes a story which connects to this idea.

Rabbi Yoḥanan sat and taught: In the future, the Holy One, Blessed be He, will bring precious stones and pearls that are thirty by thirty cubits, and He will hollow out in them a hole of ten by twenty cubits and set them in the gates of Jerusalem. A certain unnamed student sneered at him, saying: Now we do not find precious stones even of the size of an egg of a dove, and yet all of this we will find?

After a period of time that student’s ship went to sea, where he saw ministering angels sitting and sawing precious stones and pearls that were thirty by thirty cubits, and hollowed out in them were holes of ten by twenty cubits. He said to the angels: For whom are these? They said to him that in the future, the Holy One, Blessed be He, will place them in the gates of Jerusalem. Later, the student came before Rabbi Yoḥanan and said to him: Continue to interpret, my teacher, it is fitting for you to interpret, as I saw just as you said. Rabbi Yoḥanan said to him: Worthless man, if you had not seen, you would not have believed; clearly, you are mocking the statement of the Sages. Rabbi Yoḥanan set his eyes upon him, and the student was instantly killed and turned into a pile of bones.

(Translation from Sefaria)

The question is, why did Rabbi Yohanan wait to punish the student? He should have responded initially, when the student doubted the teaching of his rebbi?

The answer is that only when he returned did he express the sentiment, that he can believe it only if he can see it. And someone who is unable to go beyond what he sees, will lack all of the fundamentals of our religion. Because the continuation of our tradition is based on hearing.

And maybe we could add one more point. Why did Rabbi Yohanan call his student empty? The reason is that he did not believe in what he could not see. Therefore he did not even believe in his own neshama! Such a person is surely empty.