  This statement is based on the idea that if a proposition presently neither is nor will be true, and you step back in time, as it were, then the formerly present ‘not being true’ turns into a future ‘not going to be true’, so that from the point of view of the past, the proposition will never be true, and the corresponding state of affairs will never be the case. (This is assumed to hold at least for the past moment that immediately precedes the present.) This principle has some plausibility to it. There is some evidence that it may have been discussed in antiquity (Becker 1961, 253-5).