Not Necessarily and Not Always
If someone says, ‘Houses sell below the asking price around here,’ it is idiomatically correct to respond, ‘Not necessarily.’ ‘Not necessarily’ in this context means not always. Its meaning is not modal, but temporal: there are times when they sell below asking price, and times when they do not.
In ordinary English, the confusion of the temporal ‘always’ with the modal ‘necessarily’ is not a problem. But in more abstruse contexts, the distinction must be made. Suppose A asks, ‘Why does the universe exist?’ and receives the reply from B, ‘Because it always existed.’ This does not constitute a good reply even if it is true that the universe always existed. The reason is because a thing’s having existed at every past time gives no good answer to the question as to why it exists at all. Even if the past is infinite, the reply is defective. For even if (i) there is no past time at which the universe does not exist, and (ii) no first moment of time, one can still reasonably ask: ‘But why does the universe exist at all?’ ‘Why not no universe?’
If, however, it were said that the universe necessarily exists (cannot not exist), then (assuming the truth of the universe’s necessary existence) that would amount to a good reply to the question as to why it exists. For if X cannot fail to exist, then it makes no sense to ask why it exists.
Some atheists think themselves quite clever in objecting to theists as follows. ‘You say that God is needed to explain the existence of the universe; but then what explains the existence of God?' The short answer is that God is a necessary being, one that cannot not exist, and that to ask for the explanation of a necessary being makes no sense. This does not end the debate, of course, but it moves it from the sophomoric level up a notch to the ‘junior’ level.