An example of a rule of the natural world that a scientist can assume is always true?

One example of a rule of the natural world that scientists assume is always true is the law of conservation of energy. This law states that energy cannot be created or destroyed, only transformed from one form to another. It underpins many scientific principles and is fundamental to understanding physical processes in various fields, including physics, chemistry, and Biology. This assumption allows scientists to make predictions and develop theories about how systems behave.