  Boethius actually followed his reasoning about divine perfection to the conclusion that God exists outside time by his very nature — that God can't be temporal. For whatever has neither past nor future is not located in time. But change requires existence in time. Suppose that a turnip, aging, goes from fresh to spoiled. It also then goes from fresh to not-fresh. So first “the turnip is fresh” is true, then “it is not the case that the turnip is fresh” is true. The two cannot be true at once. So things change only if they exist at at least two distinct times. Hence, if God is necessarily atemporal — via necessary divine perfection — God is necessarily changeless, i.e. immutable. Anselm clearly picked up Augustine's and Boethius' tie between perfect being and immutability: he wrote in Monologion 28 that God alone exists in an unqualified sense and perfectly and absolutely, whereas all other things nearly do not exist at all... For because of his unchangeable eternity, it can in no way be said of (Him) because of any alteration that he existed or will exist; instead, he exists in an unqualified sense.  Nor does he exist changeably, so that he is now something that at some time he was not or will not be.  Nor does he fail to be now what at some other time he was or will be.  Rather, whatever he is, he is once and for all, all at once, and illimitably.  And since his existence is like this, he is rightly said to exist in an unqualified sense and absolutely and perfectly.