I read this article by Hawking which includes this quote " In classical mechanics one can predict the results of measuring both the position and the velocity of a particle. In quantum mechanics the uncertainty principle says that only one of these measurements can be predicted; the ob server can predict the result of measur ing either the position or the velocity but not both. Alternatively he can predict the result of measuring one combination of position and velocity." Can someone elaborate on the last sentence "Alternatively he can predict the result of measuring one combination of position and velocity" How is that supposed to work exactly? If you can know only one or the other then how do you measure the combination to predict anything?