D. How the Sacrament of Order is One

46. But now as to the fourth article [n.11], how the sacrament of order is one, I say that it is one by unity of proximate genus, as the moral virtues are said to be three according to genus: justice, fortitude, temperance. And in the first division of moral virtue these occur first; however, each of them is divided specifically into many, as was said in Ord. III d.34 nn.54-80.

47. But there is another unity between the orders, because there is a unity of order; for it is fitting to receive a lower order first before a higher one, nor yet is this order simply necessary such that, if it is omitted, nothing happens, as is plain in Gregory IX, Decretals V tit.29 ch.1, ‘About a cleric promoted by bounds’, where one gets that ‘what has been done is not to be repeated but what was passed over is to be cautiously supplied’. And consequently, it is not the case that, if a lower order has not been conferred first, nothing is done in the conferring of a higher order. But this unity of order is not that by which order or ordination is one sacrament, but is rather a prior unity, namely unity of genus [Scotus more fully in Rep. IV A d.24 q.1 n.14].