[Squidmeister]

I'd say that whether or not someone or something exists is evident by experience; if you don't have the capability to experience it in any capacity and/or in any situation (such as with sight, smell, taste, touch, touch, psychic abilities, etc.), it can be safely assumed that it doesn't exist/have the potential to exist. Conversely, if you do have the capability to experience it in any capacity and/or in any situation, it can be safely assumed that it exists/has the potential to exist.

With living, I'd say that really depends on the context you're using those words in.