Q: nil and method selection

I was reading the latest language specification on Objective-C (2.0)
from Apple and came across something that confuses me with regard to
dynamic binding and typing.

If I create a new id and assign it the value nil like this:

    id x = nil;

then call some method on it,say "methodThatReturnsDouble" as in

    double y = [id methodThatReturnsDouble];

how does the runtime know which "methodThatReturnsDouble" I am
calling, since I could define it in more than one class?

Is this true in Smalltalk also?

Thanks, Ralph