'Scripture cannot prove Scripture' denies only bare self-assertion, not divinity's marks in the text
These points, though, must be taken carefully. When we grant that sacred Scripture cannot be proved by Scripture itself, we mean only that the places where Scripture proclaims itself divinely inspired are not fit arguments to convince and persuade someone who already denies sacred Scripture's authority.
That does not stop us saying, in another sense, that Scripture proves itself: as the preceding theses showed, sacred Scripture bears many marks and arguments of divinity by which it asserts itself true and divine — just as heaven and earth proclaim and show God as their author, not by a bare spoken claim, but by the excellence of their nature and the traces of divine power and wisdom visible in them.
Or as a book claims Aristotle or Euclid as its author not from the title alone, but from the genius of the teaching, the sharpness of style, and the manner of argument — the sort that can belong only to those rare minds.
Latin
VIII. Quae tamen dextre accipienda sunt. Primo enim, cum concedimus Scripturam sacram non posse per Scripturam ipsam probari, hoc tantum volumus illa testimonia quibus Scriptura se esse divinitus inspiratam praedicat, non esse argumenta idonea quibus convincatur, et persuadeatur ille qui Scripturae sacrae authoritatem negat. At hoc non obstat quominus alio sensu Scriptura dici possit seipsam probare, quatenus, ut thesibus praecedentibus ostensum est, Scripturae sacrae insunt multa divinitatis signa et argumenta, quibus seipsam veram et divinam asserit. Non secus ac coelum et terra praedicant et ostendunt suum authorem Deum esse, non simplici aliqua voce et testimonio, sed ex ipsa suae naturae praestantia, et potentiae ac sapientiae divinae vestigiis, quae in illis notari possunt. Aut, ut liber aliquis Aristoteli, et Euclidi se vendicat, non ex solo titulo, sed ex ipsius doctrinae genio, acumine styli, et disserendi modo, qui ejusmodi est, ut non possit, nisi praestantibus illis ingeniis tribui.