486. Verse 4 And the days of Man after he beget Seth were eight hundred years; and he beget sons and daughters.

'Days' means periods of time and states in general. 'Years' means periods of time and states in particular. 'Sons and daughters' means the truths and goods that they perceived.

In many cases, the use of “eternity” in the Bible is pretty literal, simply meaning forever. This is especially true when the text refers to someone being condemned to hell or elevated to heaven. In some cases, however, “eternity” or “days of old” refers to what the Writings call the Most Ancient Church -- the first church among the earliest people, innocent souls who acted directly out of love of the Lord. Because of this association, things called “eternal” or “everlasting” usually represent the desire for good that arises from love to the Lord. If we love the Lord, then our desire will be to do His will and love as He loves. From that we will have a true desire to love and care for other people. That's the feeling described by “eternal.”