My book says a language is in NP if it can polynomially verified if a string belongs to the language with a certificate. It puts no restrictions on what the certificate can be.
For instance, for SAT, the certificate could simply be be the assignment values. We just sub them in and see if it satisfies the expression.
But can't we just let the certificate by whether the string belongs or not? So for instance, for SAT, instead of the certificate being the assignment values, we let it be whether the expression is satisfiable or not. In other words, the certificate is the "answer".
If we let the certificate be this, can't be verify any string in polynomial time? Constant in fact, because we would just be returning the certificate itself, since the certificate is the "answer" (whether the string is part of the language or not)