פן תשכח את הדברים אשר ראו עיניך
In this Parsha we return to the scene at Har Sinai, and we read again how Bnei Yisrael prefaced נעשה to נשמע. We are commanded never to forget this momentous event, as the passuk states, "השמר לך... פן תשכח את הדברים אשר ראו עיניך", beware and guard yourself lest you forget the things your eyes have seen. According to the Ramban, this is counted as one of the מצות לא תעשה, the negative commandments.
It is interesting to note that the passuk
We can answer as follows: the passuk states "וכל העם רואים את הקולות", and all the people saw the sounds. Our Sages tell us that what Bnei Yisrael experienced at Har Sinai was nothing short of spiritually induced synesthesia: they saw the sounds and heard the sight. Similarly our Sages tell us that they were able to point at G-d and say "זה קלי ואנוהו", this is my G-d and I will glorify Him.
There is a deep and profound message in this concept. As Rav Chaim of Volozhin explains, it is human nature and commonly accepted that of the five senses which G-d has granted man to make him an intelligent and understanding being, eyesight is the most reliable. There is not a shadow of doubt in one’s heart as to the validity of what he has seen with his own eyes. This is not the case with any of the other senses. For example, a person might doubt something he has heard, as our Sages tell us "לא תהא שמיעה גדולה מראיה", the hearing should not be greater than the sighting. But Bnei Yisrael, who uphold the belief of their forefathers, know that what their eyes see is not necessarily the truth. G-d Himself, about Whom the Rambam writes that all things in existence in the heavens and the earth and anything in between are in existence solely because of G-d’s existence, cannot be seen as He is. As the passuk states, "כי לא יראני האדם וחי", for no human can see Me and live. In other words, our eyes do not see the inner, quintessential existence; for what does man really amount to if not his soul, the divine part breathed into him by the Creator—and the soul is not even visible! Nor can our eyes see the ministering angels performing G-d’s commands in His world, or the demons and spirits. But our fathers have told us about all this and so we’ve learned from our holy scholars in each generation. The Ramban writes that our faith is grounded in the tradition that has passed down from generation to generation, even though these things are not visible to the human eye.
When Bnei Yisrael stood at Mount Sinai they attained such a high level that they saw the sounds, meaning that that which they heard was so real and tangible to them that it was as if they had actually seen the words in a concrete and absolute way. Likewise they heard what their eyes beheld, meaning that all things visible to them—the material world—were removed from their consciousness and feeling, and were merely like hearing (and thus not absolutely reliable).
Thus we can understand the following Gemara
R’ Yochanan once sat and lectured: the Holy One, Blessed is He, will one day bring precious stones and pearls which are thirty amos by thirty amos, and He will cut out from them an opening of ten amos by twenty amos, and He will place them at the gates of Jerusalem. A certain student mocked him for discussing such incredible occurenes. The student said: now, we cannot find precious stones and pearls even the size of an egg of a small dove, can stones of such an immense size ever be found? After a time the student’s ship set sail upon the sea, and he saw ministering angels sitting and sawing precious stones and pearls that were thirty amos by thirty amos, and ten amos by a height of twenty amos was cut from them
,. The student said to the ministering angels, For whom are these? They told him that the Holy One Blessed is He, will one day place them at the gates of Jerusalem. The student came back before R’ Yochanan and said to him, My Master! Continue to lecture! You are fit to lecture. Just as you said, so I saw. R’ Yochanan said to him: You empty person! Had you not seen it yourself would you not have believed it? You are one who mocks the words of the Sages. R’ Yochanan set his eyes upon him and the student became a heap of bones.
It is interesting to note that the student was not punished for mocking the words of his teacher R’ Yochanan, but rather was punished when he returned to R’ Yochanan to confirm what he had said. This warrants explanation, but based on what we’ve learned we can understand. When the student came back to R’ Yochanan and expressed his amazement and excitement for what he saw, he said “just as you (R’ Yochanan) said, so I saw.” By saying this he showed that through his seeing with his own eyes he believed what R’ Yochanan had said, and therefore he retracted his mockery of R’ Yochanan’s initial words to him. R’ Yochanan got angry with him and said “You empty person! Had you not seen it yourself would you not have believed it?” R’ Yochanan was rebuking his student for not believing the words of the Torah. Is it our eyes—what we see before us—that determine whether we believe something, or the Torah which we heard with our ears that determines what we believe? For this the student was punished, because someone of his stature (and surely he was of a great caliber—he had witnessed ministering angels and even conversed with them), should be able to see the sound, to believe that which his ears hear, and know it to be true as if he had seen it.
The student’s punishment was measure for measure; in it we see a reminder of his transgression. To the naked eye, a human being is merely a heap of bones. Yet we know that his life comes solely from the power of his divine soul which sustains him and makes him alive. Because the student valued only that which his eyes were able to see, he was turned into a heap of bones, measure for measure. This is why R’ Yochanan said to him You empty person! Do you not admit that you have a soul in you although it cannot be seen? Why can’t you “see” what is heard and “hear” what you see?
The Gemara tells us
Rav Sheishes was full of light (i.e. blind). Everyone was going to greet the king. Rav Sheishes rose and went with them. A certain Sadducee met [Rav Sheishes], and said to him “whole pitchers go to the river. Where do broken [pitchers] go?” [Rav Sheishes] answered him, “come and see that I know more about the king’s procession than you do.” The first troop passed. When it became loud, that Sadducee said to [Rav Sheishes], “the king has come!” Rav Sheishes said to him, “he is not coming.” A second troop passed by. When it became loud, that Sadducee said to Rav Sheishes, “now the king is coming!” Rav Sheishes said to him, “the king is not coming.” A third [troop] passed by. When it subsequently grew quiet, Rav Sheishes said to him, “It is now certain that the king is coming.” That Sadducee asked him, “how do you know this?” He answered him, “for the royalty of earth is a reflection of the royalty of Heaven. Hence I knew that the king would come when it grew quiet after three troops passed by because it is written regarding the royalty of Heaven: ‘Go out and stand on the mountain before Hashem. And behold, Hashem was passing, and a great powerful wind, smashing mountains and breaking rocks, went before Hashem; Hashem was not in the wind. After the wind came an earthquake; Hashem was not in the earthquake. After the earthquake came a fire; Hashem was not in the fire. After the fire came a still, thin sound.’” When the king finally came, Rav Sheishes began blessing him. That Sadducee said to him, “someone you cannot see you are blessing!” What became of that Sadducee? Some say that his colleagues painted his eyes. And some say that Rav Sheishes set his eyes upon him and he became a heap of bones.
This too teaches us a profound lesson, that what our eyes see does not determine the absolute truth, rather what we hear does. Rav Sheishes who was blind not only understood spiritual matters better than the Sadducee but understood earthly matters better than him as well. When the Sadducee did not learn a lesson but stayed set in his ways to value “seeing” as the uppermost sense and to disregard Rav Sheishes by asking him “someone you cannot see you are blessing!” he was punished measure for measure and his eyes were painted or he became a heap of bones.
In the future we will merit "כי עין בעין יראו בשוב ה' ציון וראו כל בשר יחדיו כי פי ה' דבר" with their own eyes they will see that Hashem returns to Zion, and all flesh together will see that the mouth of Hashem has spoken. Similarly, we daven" "והוא ישמיענו ברחמיו שנית לעיני כל חי לאמר and He will let us hear, in His compassion, for a second time in front of the eyes (i.e. in the presence) of all living, saying… Do our eyes see what we hear? Eyes see and ears hear! But in future times we will once again attain the level we attained at Har Sinai when we saw the sounds.
Now it is very clear how we could be commanded beware and guard yourself lest you forget the things your eyes have seen.
 דברים ד': ט'
שכחת הלאוין לדעת הרמב"ן מצוה שניה
 שמות כ': ט"ו
 מכילתא פרשת יתרו שם
 נפש החיים שע"ג פי"א בהגה"ה . See also בני יששכר on חדש סיון
 ראש השנה כ"ה ע"ב
 הלכות יסודי התורה, פ"א ה"א
 שמות ל"ג: כ'
 דברים ד': ט'. See also the introduction to ספר החינוך
For further insight see the משך חכמה on פרשת חקת and the מאור עינים on פרשת בשלח where each of them writes that we must ensure the power of hearing overpowers the power of seeing.
 בבא בתרא ע"ה ע"א
 ברכות נ"ח ע"א
 ישעיה נ"ב: ח'
 ישעיה מ':ה'
 קדושה למוסף של שבת ויו"ט