Ordinary experience and scientific investigation give us the impression that some things could or could not have been otherwise. Had we been more responsible, this special issue would have been published earlier. Had we decided to do so, we could have drunk tea rather than coffee for breakfast this morning. But no matter how different we had behaved in the past, 2 + 2 = 4 would have not been otherwise. And for all we know, only if the laws of nature had been different, two fermions could have occupied…
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