 A world line is “the collection of points on a space-time (Minkowski) diagram which represents the history of an object”, and a conserved quantity “is any quantity universally conserved according to current scientific theories” (Dowe 1992: 210; see also Dowe 1995 and 2000). Examples of conserved quantities mentioned by Dowe are mass-energy, linear momentum, angular momentum, and charge. Salmon regarded with favor Dowe’s proposal and accepted a slightly modified version of it, according to which