A principle in science is something which is not derived but is simply stated without proof.
Examples:
- Heizenberg's Uncertainty Principle is not a principle being derived from other quantum mechanical ideas. It is increasingly referred to as "Heizenberg's Uncertainty Relations."
- The Principle of Microscopic Reversibility is, surprisingly perhaps, not believed to be a consequence of the Second Law of Thermodynamics and therefore constitutes a true principle.
- The Principle of Equivalence is a straightforward principle: It states that it is impossible to decide whether one is accelerating or rather is in a gravitational field.
Einstein gave the example of being inside a chest, hanging in space from a rope. Some being pulls on the rope with a constant force. Quickly the chest reaches unheard of velocities. The person inside the chest, initially floating in zero gravity, "falls" to the floor. A hammer and a feather fall and hit the ground at the same time. Should the man climb out of the chest and see the rope pulling the chest upward he is at liberty to say, "That's why the chest is not falling - it is suspended from this rope."