Einstein's Postulates

1) There is no preferred inertial frame. If you feel at rest then you are at rest as much as anyone can be.
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2) The speed of light in a vacuum is measured to give the same value, c, by any inertial observer.

Operational Definitions

All quantities defined in terms of operations
- Length: lay out meter sticks
- Time: look at a clock
- Simultaneity: If I am at the midpoint of the events and I see them at the same time then they happened at the same time.