Argument based on speculation

I was wondering if it is acceptable to argue a point that cannot be proven. For example; Person A states that majority of people on this earth have never stolen anything in their life. Would it be acceptable to argue against Person A using speculation as to why the majority of people have stolen at least once(human nature, etc)?

Some knowledge is provable, such as science, or statistics (e.g. your example).

However, other statements are true within themselves (e.g. "All bachelors are unmarried).

The latter statement needs not rely on evidence, as all bachelors are unmarried by defenition.