This is a question more about how we can discuss about objects which no longer exist. For example, let's say that Socrates no longer exists (ignore any religious side of this and consider Socrates as simply being a physical entity) how come we can still use past tense language to discuss about him?
If we say that existence is a mandatory property of an object then Socrates IS not an object, however Socrates WAS an object, as Socrates does not exist, but has existed. This presents several issues for me, how can Socrates, not being an object, be an element of my domain of discourse when a set is defined as a 'collection of objects'?
How can Socrates be a referent in my language when Socrates is not an object? This would suggest that 'Socrates' does not denote anything, which it clearly does.
How can we have sentences and relations pertaining to Socrates when he does not exist as an object?