That's what they want you to believe.
Necessary is something that absolutely needs to happen. Unnecessary is something that can happen, but doesn't have to happen. More opposite to necessary, is forbidden, since that means it absolutely shouldn't happen.
I think it's not impossible, because impossible means that it's not possible for it to happen; but that's not really opposite to necessary, as necessary doesn't say something about the probability of something to happen, but more about how likely you want it to happen.
Necessary is more a subjective word.
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