If we know that object a has property P, we can certainly infer
that something has this property. If we know that Tom is both an
actor and a waiter, we can infer that some actors are waiters.
In existential generalization, then, we replace a name with a
variable and add an existential quantifier. When we do so, we
must be sure to place the quantifier at the beginning of the
statement, so that the entire statement falls within its scope.