 For example, if a planet is not moving in quite the smooth ellipse that it ought to follow a) if Newtonian mechanics were correct and b) if there were nothing but the sun and the planet itself to worry about, then the positive heuristic of the Newtonian programme bids us look for another heavenly body whose gravitational force might be distorting the first planet’s orbit. Alternatively, if stellar parallax is not observed, we can try to refute this apparent refutation by refining our instruments and making more careful measurements and observations.