KJV ... Only take heed to thyself, and keep thy soul diligently, lest thou forget the things which thine eyes have seen, and lest they depart from thy heart all the days of thy life: but teach them to thy sons and sons' sons.
AMP. ... Only take heed, and guard your life diligently, lest you forget the things which your eyes have seen, and lest they depart from your [mind and] heart all the days of your life: teach them to your children and your children's children.
The Message Bible ... Just make sure you stay alert. Keep close watch over yourselves. Don't forget anything of what you've seen. Don't let your heart wander off. Stay vigilant as long as you live. Teach what you've seen and heard to your children and grandchildren.
This is talking about "what you lose from your eyes is going to depart from your heart. " This principle carries over to the New Testament also. If we are not looking at the Word and gleaning from it, not only the knowledge and information, but the revelation of it will be lost, and we'll never be able to walk through the things of God the way we are supposed to walk.
Reverend Robert W. & Mary C. Butler