IX. Although natural liberty agrees in essentials with the liberty of man constituted in other states, still it differs greatly in accidentals. For the liberty of glory in blessedness is not to be able to sin (non posse peccare). The liberty of sinners in the state of sin is not to be able not to sin (non posse non peccare). The liberty of believers, in grace, is to be able to sin, and not to sin (posse peccare et non peccare). But the liberty of Adam was to be able not to sin (posse non peccare). The first was an absolute incapability of sinning; the second an incapability of acting well; the third the power of sinning and of acting well. The fourth was the power of not sinning. Augustine explains this excellently: “We must diligently and attentively examine if these good things differ, to be able not to sin (posse non peccare), and not to be able to sin (non posse peccare), to be able not to die, and not to be able to die, to be able not to leave the good, and not to be able to leave the good. For the first man was able not to sin, not to die, not to leave the good” (Admonition and Grace 12* [33] [FC 2:285; PL 44.936]). And afterwards: “Therefore the first liberty of will was to be able not to sin (posse non peccare), the last will be much greater, not to be able to sin (non posse peccare). The first immortality was the power of not dying, the last will be much greater, the incapability of dying. The first was the power of perseverance, the power to not desert the good, the last will be the happiness of perseverance, the want of power to desert the good” (ibid., pp. 285–86).
Francis Turretin, Institutes of Elenctic Theology, trans. George Musgrave Giger, ed. James T. Dennison Jr., vol. 1 (Phillipsburg, NJ: P&R Publishing, 1992–97), 8.1.9 (p. 571).