 [impossible] is used in one way of whatever cannot be true now or in the future or in the past; and this is ‘impossible per se’ … It is used in the other way of whatever cannot be true now or in the future although it could have been true in the past … and this is ‘impossible per accidens’. Similarly, in case something cannot be false now or in the future or in the past it is said to be ‘necessary per se’ … But it is ‘necessary per accidens’ in case something cannot be false now or in the future although it could have been [false] in the past (Kretzmann 1966, p. 41).