Einstein's starting premise for his work on the special theory of relativity was that the laws of physics are the same for all observers in uniform motion. This is known as the principle of relativity.

This simple, yet profound, assumption led to two postulates that formed the foundation of special relativity:

1. The laws of physics are the same for all observers in uniform motion. This means that no matter how fast you're moving at a constant speed, the laws of physics will remain the same. For example, a ball dropped inside a moving train will fall straight down to the floor, just like a ball dropped on the ground.

2. The speed of light in a vacuum is the same for all observers in uniform motion. This means that no matter how fast you're moving, even at speeds approaching the speed of light, you will always measure the speed of light as the same constant value. This was a radical departure from classical physics, which assumed that speed is relative.

These two postulates led to several surprising conclusions, including:

* Time is relative: Time is not absolute but depends on the observer's frame of reference.

* Length contraction: The length of an object appears to shrink in the direction of motion when observed from a moving frame of reference.

* Mass-energy equivalence: The famous equation E=mc² demonstrates that mass and energy are equivalent and can be converted into one another.

Einstein's special theory of relativity revolutionized our understanding of space, time, and gravity, paving the way for modern physics.