But why do the atoms have to be infinite? Couldn’t there be, say, exactly ten billion atoms bouncing around in an infinite emptiness? In answer to this, the ancient atomists could invoke a rule which is sometimes called the principle of sufficient reason. It states that there has to be some good reason or explanation for each feature of the universe. In the present case, atoms must be infinite because there is no reason why there would be any particular finite number of them.
