There has been a demise of the crown.

At the instant of the demise the next heir became our lawful sovereign.

We consider the Princess of Orange as next heir; and we hold that she ought, without any delay, to be proclaimed, what she already is, our Queen.
The Whigs replied that it was idle to apply ordinary rules to a country in a state of revolution, that the great question now depending was not to be decided by the saws of pedantic Templars, and that, if it were to be so decided, such saws might be quoted on one side as well as the other.

If it were a legal maxim that the throne could never be vacant, it was also a legal maxim that a living man could have no heir.

James was still living.