Think of the derivative of a function, such as x2 + 7. The derivative is an absolute: 2x; just as a chunk of data run through md5 results in an exact checksum. However, if you only know the derivative (or the checksum), you can't go backwards because if one reverses the derivation, the result is x2 + C, but C is unknown in this case. Information is lost in the conversion that cannot be recreated.
