Information Evaluation

Chapter 8

When Reported Information Is Second Hand

8.1. Introduction

In order to estimate the current state of the world, a rational agent seeks to acquire information about that state, using sources of various types: physical sensors, human observers, Websites, newspapers, etc.

The agent receiving this information then asks himself the question of whether to accept it as new beliefs or to reject it. Obviously, he can accept a piece of information as a new belief if he knows that the source providing it always gives information which is true. Similarly, if the agent knows that a source systematically provides false information, then he can accept as a belief the opposite of any information produced by that source.

For example, suppose I am interested in the flow rate of a river fed only by melt water from a glacier. In order to estimate the flow rate of that river, I telephone a friend who lives near the glacier. That friend tells me that the temperature around the glacier has increased dramatically in the past few months. If I trust that friend to give accurate climatic information, then I can accept the fact that the temperature in the environs of the glacier has increased, and conclude from this that the flow of the river has also increased. Now suppose that instead of phoning my friend, I read the weather report in my newspaper, which says that the temperature in the mountains has risen. If I know that the weather reports in that newspaper are always wrong, then I can conclude ...