The reason I say is that it's a calculation... so if you can calculate the Public key from the Private and know the calculation to do so, then in theory, you should be able to adjust the variables within the calculation to prove the opposite.

E.g. a = b*c where a is the public key, b is a calc. and c is the private key would lead to c = a/b, therefore allowing us to calculate the Private Key from the Public.

I believe that there's an algorithm that exists linking one with the other, but if this is known in one direction, then the reverse will be calculable.