Membership problem of Recursive languages are decidable.
Let be a recursive language and be the Turing Machine that accepts it. For string if then halts in final state. If then halts in non-final state. (halts always!). That’s why Recursive languages are decidable.
My question is in same logic, why finiteness, emptiness is undecidable? Don’t want any concrete proof. I just want brief concepts like my approach.