The Institutes of Caius, (l. ii. tit. ix. x. p. 144 - 214,) of Justinian, (l. iii. tit. xiv. - xxx. l. iv. tit. i. - vi.,) and of Theophilus, (p. 616 - 837,) distinguish four sorts of obligations — aut re, aut verbis, aut literis, aut consensû: but I confess myself partial to my own division.