Full Idea
If the present is the limit of the past, and the limit of the past has passed away together with that of which it is the limit, the present no longer exists. And if the present begins the future, which doesn't exist, the present does not yet exist.
Gist of Idea
If the present is just the limit of the past or the future, it can't exist because they don't exist
Source
Sextus Empiricus (Against the Physicists (two books) [c.180], II.201)
Book Reference
Sextus Empiricus: 'Against the Physicists/Against the Ethicists', ed/tr. Bury,R.G. [Harvard Loeb 1997], p.311
A Reaction
If I mark a line on the ground where the wall will begin, the limit seems prior to the object. The gun starts the race, but is not part of it. That said, I cannot think of any more mysterious entity than the present moment. It isn't a line or a bang.