Philosophers in the 17th and 18th century questioned if time was real and absolute, or if it was an intellectual concept that humans use to understand and sequence events. These questions lead to realism vs anti-realism; the realists believed that time is a fundamental part of the universe, and be perceived by events happening in a sequence, in a dimension. Isaac Newton said that we are merely occupying time, he also says that humans can only understand relative time. Relative time is a measurement of objects in motion. The anti-realists believed that time is merely a convenient intellectual concept for humans to understand events. This means that time was useless unless there were objects that it could interact with, this was called relational time. René Descartes, John Locke, and David Hume said that one's mind needs to acknowledge time, in order to understand what time is. Immanuel Kant believed that we can not know what something is unless we experience it first hand.