serve as? معنی اش در جملات زیر

Question

A flat stone served as a table.

Neither plot can serve as an adequate guide to the middle.

Aristotelian moral philosophy cannot serve as an adequate guide to the moral order.

Whether appetite uninfluenced by experience with food could serve as an adequate guide to physiological needs was one of...

It is hoped that this volume will serve as an adequate guide to laboratory techniques and their applications for research workers,...