Either Bill Clinton or George W. Bush was president when the World Trade Center towers were destroyed. But Bush was not president at that time. Therefore, Bill Clinton was president when those buildings were destroyed.

Informal logic is the study of logic and logical thought outside of a formal setting. It's a contemporary way to explain, systematize, assess, and teach arguing for practical purposes.  Informal logic systems can vary in a number of ways. They may incorporate formal, rhetorical, dialectical, and other methods of analysis. They may also propose unique approaches or mix methods that they borrow from other systems.  Informal logic is the development and justification of practical guidelines for recognizing, identifying, and displaying the reasoning expressed and invited in arguments. It... Show more

Informal logic is the study of logic and logical thought outside of a formal setting. It's a contemporary way to explain, systematize, assess, and teach arguing for practical purposes. 

Informal logic systems can vary in a number of ways. They may incorporate formal, rhetorical, dialectical, and other methods of analysis. They may also propose unique approaches or mix methods that they borrow from other systems. 
Informal logic is the development and justification of practical guidelines for recognizing, identifying, and displaying the reasoning expressed and invited in arguments. It combines the study of argument, evidence, proof, and justification with an instrumental outlook which emphasizes its usefulness in the analysis of real life arguing. 

Here are some examples of informal logic:
Premises: Nikki saw a black cat on her way to work. At work, Nikki got fired. Conclusion Black cats are bad luck.
I am at the bus stop at 7 am when the bus usually picks us up.: The bus is not here, and it is 7:05 am. The bus is running late. 
Informal logic is associated with informal fallacies. 

Here are some basic concepts of informal logic:

Context, content, and delivery: Informal logic considers the context, content, and delivery of an argument.
Fallacies: Informal logic recognizes fallacies that can destroy dialogue, and teaches how to avoid or use them. A weak analogy fallacy is a part of informal logic. It occurs when comparing two similar things, and drawing a conclusion from the comparison.
Informal fallacies: Informal fallacies are mistakes in reasoning that are relatively common, but neither formal nor formally characterizable in any useful way.
Argumentation theory: Argumentation theory is the study and research of informal logic, fallacies, and critical questions as they relate to every day and practical situations.
Argumentation schemes: Argumentation schemes are stereotypical patterns of inference that represent the abstract structure of the most common types of natural arguments.
Argumentation: Argumentation is the basis of logic in that it presents a series of statements or premises that help support an overall claim.

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