Scienter is a Latin adverb meaning 'knowingly'. Many Latin legal phrases change parts of speechiness over time: e.g., sub poena is a prepositional phrase what got nouned, ignoramus was a verb to noun, habeas corpus a sentence to noun.

FWIW, most of the ghits on the first page or so of Google books (link) are sans with.

Ceci n'est pas un seing.