Aristotle's Rules of Logic.

[inibo]

You may ignore this, I'm just putting it here so it will be handy. Some day I may talk about it, but not today.[T]o help with valid reasoning Aristotle came up with three rules of logic:

• The Law of Identity, or A is A.
  
  
• The Law of Non Contradiction, or A is not not-A.
  
  
• The Law of the The Excluded Middle, each thing is either A or not A

The Law of Identity, or the Law of Intrinsic Nature, says that an existent is what it is, that is, an automobile has the nature and properties or attributes of an automobile.

The second Law says that something, such as an automobile, cannot both have the nature and properties of that something, and yet also have the nature and properties of something else (in the same respect), such as those of an aircraft.

The third Law says that each existent in existence has either the nature and properties of something, such as an automobile, or the nature and properties of something else, with nothing in between. ~From Concepts to Experiential Consciousness:Philosophical Underpinnings of Research in Databases and Computer Science, by J. Bradley
I'm not so sure about that third one, but I haven't really given it much thought.