Then the Lord God said, “See, the man has become like one of us, with his knowledge of good and evil. He must not be allowed to stretch his hand out next and pick from the tree of life also, and eat some and live forever.” [Genesis 3:22 (JB), slightly amended].
In a recent comment I observed that there was reason to think that death must have been a feature of the world before the Fall. Here’s further reason to think so. In the first place, Adam was told that he would die on the day he ate from the tree of the knowledge of good and evil—something which did not literally happen, and which has always been understood as referring to his spiritual death (a consequence of his sin). But if it referred to his spiritual death, then it does not have reference to his earthly life. Then later (see above) the Lord revokes Adam’s access to the tree of life, from which he would have had to eat in order to live forever. The obvious inference is that without doing so he would not live forever, and consequently he was mortal but with a calling to an eternal life which he had just lost. As the notes to the JB say, “immortality was a pure gift of God which man’s disobedience forfeited.”
It’s important to understand this correctly because man does not by nature have any right to God’s presence, nor does he have any power to see God or save himself by his own strength. This is the error of the Pelagians. See this post (among other things) for a little more.