In A Plea for Captain John Brown, Thoreau writes:

I have no respect for the penetration of any man who can read the report of that conversation, and still call the principal in it insane. It has the ring of a saner sanity than an ordinary discipline and habits of life, than an ordinary organization, secure.

I have not read anywhere that secure is used as a noun. Is the word used here as a noun? What does secure mean here?