Despite the close relationship between math and physics, they are not synonyms. In mathematics objects can be defined exactly and logically related, but the object need have no relationship to experimental measurements. In physics, definitions are abstractions or idealizations, approximations adequate when compared to the natural world. For example, Newton built a physical model around definitions like F = m a based on observations, leading to the development of calculus and highly accurate planetary mechanics, but later this definition was superseded by improved models of mechanics. Mathematics deals with entities whose properties can be known with certainty. According to David Hume, only in logic and mathematics statements can be proved (being known with total certainty). While in the physical world one can never know the properties of its beings in an absolute or complete way, leading to a situation that was put by Albert Einstein as "No number of experiments can prove me right; a single experiment can prove me wrong."