The mind once enlightened cannot again become dark.

Paine is making a point about the one-way nature of understanding: once you have truly seen why something is so, you cannot sincerely go back to believing otherwise. Ignorance can be left behind the way a town is left behind, and the road back does not lead to the same place. The idea belongs to the Enlightenment faith in reason and public education, which Paine championed in his writings against hereditary privilege and inherited dogma. For him, tyranny depended on people not knowing, so a population that had learned to think for itself would be very hard to rule by fear or mystery.

It is a hopeful claim, and probably too tidy, since people can certainly be pressured into pretending they have forgotten what they know. But it rings true of real insight, which tends to stick.

A person today might apply it by learning how a few of the things around them work, such as how headlines are written to provoke, or how interest compounds on a debt. After that, the old naive reaction is gone, and they can make calmer choices without having to remind themselves to.