Дварим 4:39 Комментарий: Sforno, Or HaChaim, Rabbeinu Bahya & Rav Hirsch

וידעת היום, it behooves you therefore to understand how these miracles confound any theological theories that are not in agreement with Jewish tradition.

וידעת היום, "and you should know this day, etc." What is it that the people should know about on this day rather than on any other day? What knowledge were they to store up in their hearts? Perhaps Moses referred to the need to subdue the evil urge which lurks within all of us. Moses may have referred to 10 such pieces of information which the Israelites were to store up in their hearts.

והשבות אל לבבך, “you will impress it upon your heart,” i.e. it is a positive commandment to know the Lord and to research the fact that He is unique, one of a kind. It is not enough to accept this merely as “something handed down by our forefathers,” but it must become something that is the result of our examining the plentiful evidence in our world which makes this quite clear.