But to understand that the physicist actually explains what aspects of nature, we first need to examine the approach of physics to the natural phenomena.

Aristotle argues to proof his claim about sphericity of the Earth: Travelers going south see southern constellations rise higher above the horizon, and the shadow of Earth on the Moon during a lunar eclipse is round.

Physics studies the physical phenomena. But it can also have an effect on expanding the boundaries of observation. The physicist also experiments to determine the validity of theories. The experiment can take place with or without assigning treatments to the subjects. Though, we call the second manner an observation.

For example, one of the most important discoveries on motion, the theory of classical relativity (Galileo-Newtonian), states that all objects that move at a constant velocity in a straight-line will follow the same consistent laws of nature. An experiment that can prove the validity of this theory, at least in the scope which the theory explains, is an imaginary experiment. We assume that we are in a spacecraft which moves in a straight-line at a constant velocity. We also assume that the rocket does not have any shaking. While the chamber doesn’t have a window outside, there is no way to know if we are stationary or moving. In fact, the velocity of a moving object can only be measured relative to a reference object. As a result, it’s not possible to distinguish between two observers who travel at different speeds in different directions, and discovering the laws of nature independently.