New information can be attained by an agent in two broad ways: sensory input and information processing.

The first type involves gathering information via one or more sensory organs or apparatus, such as seeing the color of a fruit, hearing a new song, or determining the weight of a box by hefting it. A bat gathers information about the spatial layout of the world and the location of prey via sound waves. Some organisms use electrical fields. This is called empiricism.

The second type involves performing processing on information already acquired via the senses to derive new information. If one guesses the weight of the box before lifting it based on other evidence, such as its shape, size, and any labeling, this is an example of the second way an agent can attain new information. This is called rationalism.

Problems can arise with either form of information acquisition, lowering the correlation between the representation and the referents and thus the truth value of that information. Any engineer will tell you that every sensor has noise; it's just a matter of how much. As we age, the quality of sensory input deteriorates as our eyes and ears don't function quite as well as they once did.

And all sorts of things can go wrong with the way information is processed. Illusions are one example. With optical illusions, the apparatus set up to interpret the incoming data one way actually gets it all wrong.

Poor reasoning is another. New information is derived from making inferences based on other information. These existing pieces of evidence have various weights associated with them, depending on their importance in making the inference and their reliability.

Information from less reliable sources should be weighted lower. Information that has less relevance in making the correct inference should be weighted lower or discarded altogether.

How else do we continually try to insure that our internal representations of the world are as close to possible to the real world?

In The Demon-Haunted World Carl Sagan has a chapter entitled "The Fine Art of Baloney Detection", which should be required reading for anyone old enough to read. He discusses logic fallacies and rhetorical devices intended to subvert or obfuscate the truth (e.g., straw men and slippery slopes). Knowing these devices is part of assembling what Sagan calls a baloney detection kit, which is basically the cognitive apparatus for discerning information with a high truth value from information with a low truth value. Besides the techniques of logic and rhetoric, Sagan offers nine key inclusions into the toolkit, paraphrased as follows:

• Always seek independent confirmation
  
• Encourage substantive debate
  
• Seek alternative explanations or hypotheses
  
• Avoid overattachment to current explanations
  
• Seek to quantify
  
• Look for weak links in chains of reasoning
  
• Use Occam's Razor
  
• Develop falsifiable hypotheses
  

Some of these sound like guidelines for scientists, but they can, and should, be used on a daily basis by anyone. Some truths are simply difficult to ascertain, while others are intentionally muddled by those whose goals might be achieved via deception. We all have to assess the validity of the torrent of information we are subject to every day, from the words of friends, family, and politicians, to the words and images in the media.

Accepting information uncritically is a poor path, but so is radical skepticism. The goal is to develop critical thinking skills which allow for the strongest, most valid inferences to be drawn, so that what you know conforms as much as possible to what really is.

The next section will discuss what it means to value truth and what sort of behavior follows from holding truth as a primary value.