 First, there are too many. Were we inertial observers, then there would be a hyperplane of simultaneity orthogonal to our world line (and uniquely definable from our world line using the Minkowski metric). But every inertial world line defines such a hyperplane. Choosing a particular one, according to Dieks, “is tantamount to augmenting the structure of Minkowski spacetime” (Dieks, p. 5).