They must have mass. This is not a particularly stringent or discriminatory requirement.
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A probability must needs be a number between 0 and 1 (often expressed as 0% and 100%), inclusive.
x and y must both be positive.
Weigh your self. For some reason people seem to think Stone and KG are your weight however weight does not exist, it only exists because of gravity hence must be measured in Newtons (N). See the link if you want to work out your weight from your mass.
he believed if you can doubt it, it must not exist.
A country must be free to exercise sovereinty in handling its own affairs.