6.321 “Law of causality” – that is a generic name. And as we say there are minimum laws in mechanics, – such as that of the least action – so there are in physics laws of causality, laws of the causality form.
Laws like the one discussed in 6.32, presumably.
Black (p. 346) quotes Mach (Mechanics, p. 460) saying that: “Maupertuis really had no principle, properly speaking, but only a vague formula, which was forced to do duty as the expression of different familiar phenomena not really brought under one conception.” On p. 551 Mach adds that Euler changed the principle (i.e. the law of least action) “into something new and really serviceable.”