Look at fallen nature without its guilt: pure nature differs as the naked from the stripped, negation from privation
To make it easy to see what such a state of nature includes, they tell us to look at fallen nature: whatever we find in it apart from the guilt of sin, they say, belongs to pure nature.
Their set rule is that pure nature differs from fallen nature as the naked man differs from the man stripped. The two differ only in that the one has lost what the other never had. So pure nature is like a negation, fallen nature like a privation.
Latin
XIX. Ut vero facile intelligatur, quid hujusmodi naturae status comprehendat, naturam lapsam intueri jubent, in qua quicquid praeter peccati reatum deprehendimus, hoc ad puram naturam pertinere profitentur. Quippe Regula solennis eorum est, naturam puram a natura lapsa discrepare sicut nudum a spoliato, qui non aliter differunt, nisi quod hic amiserit, quod alter nunquam habuit: ut proinde pura natura se habeat instar negationis, lapsa instar privationis.