Someone in a closed box canot tell whether he is sitting at rest in the earth's gravitational field or bein accelerated by a rocket in free space.

Summary

This quote describes the concept of an observer's inability to distinguish between two different scenarios based solely on their internal perspective. It suggests that an individual enclosed in a box is unable to discern if they are at rest under the influence of Earth's gravity or being propelled by a rocket in the vacuum of space. In both situations, the experience of gravity would be the same, making it difficult for the observer to determine their actual condition without external references or information. Such an example emphasizes the importance of external frames of reference in understanding one's true state.