  The Aristotelian distinction between arts (artes) and sciences (scientiae) serves as the starting-point for Zabarella’s philosophical system. At the beginning of his Opera logica, Zabarella draws a distinction between the eternal world of nature and the contingent human world. From this distinction he proceeds to two corresponding kinds of knowledge, and two distinct methods of defining them. Zabarella maintained that, properly speaking, sciences are concerned with the eternal world of nature and thus are contemplative disciplines, whereas arts are concerned with the contingent world of human beings and thus are non-contemplative, being productive instead. The sciences in the proper sense of that term, as pertaining to demonstrative knowledge, are limited to those disciplines that deal with the necessary and eternal or with what can be deduced from necessary principles. Zabarella notes that Aristotle requires two kinds of certainty from science. One is in the knowable things, which are necessary as such (simpliciter); the other is in the mind of the scientist, who must be absolutely sure that things cannot be otherwise. The necessity involved is therefore both ontological, with respects to the objects known, and cognitive, with respect to the knowing subject (Kessler 1998, p. 837).