Sforno on Deuteronomy 4:9/ספורנו על דברים ד׳:ט׳

1רק השמר לך, even though I have said that you are wise people in the eyes of the gentile nations, be careful not to absorb the views of the wise men of those nations who deny the existence of G’d the Creator and His ability and His supervision of the universe He has created. They may try to convince you with all kinds of intelligent sounding theories.

2פן תשכח את הדברים אשר ראו עיניך, The reason why G’d has cautioned you to be so much on your guard is that you might forget the evidence of your own eyes, at Mount Sinai, as well as in Egypt and at the Sea of Reeds, and as a result be misled by your mental eyes when interpreting the Torah. This was the purpose of miracles which challenged your intellect, to show you that commonly held concepts are not necessarily true. [perhaps the author has in mind the first such miracle Moses experienced when he observed with his physical eyes that the burning bush did not burn up and turn into ashes. The Israelites, when observing the spectacle of fire within the ice of the hail, the seventh plague, collectively experienced another challenge to the concept that fire and water are totally incompatible with one another. Ed.]

3והודעתם לבניך, you must therefore relate these experiences you had with your own eyes to your children who have not seen such miracles.