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Wednesday, March 21, 2012

The Nature of Discourse and Argument Part 5

Types of Logic

Logic is the process of making reasons. It is important to distinguish this from making excuses or for attempting to explain something. It has three main forms: Deduction, Induction, and Abduction. These have varying levels of certainty and are useful in different circumstances.  In all cases the argument (remember what I mean by this term) can either be well constructed with each step leading to the next or not well constructed with missing or incorrect connections between ideas.  The measure of these to states is called Validity, which deals with the form of the argument.  Invalid arguments are most often either incomplete or fallacies.  

Deduction: Proceeding only from known information to that which can be directly derived from it.  This deals with the concept of necessary truth, that is things that, given certain other things, must be true.  It is through the conditions of validity that we gain our grasp on what must be necessarily true (or false) based on premises. If you have a valid argument with true premises than the conclusion must be correct.  The technical term used for the conclusion is "sound" for correct or "unsound" for incorrect.   Mind your P's and Q's 

Induction:   Making a  "reasonable" extension from what is known to what is unknown.  Induction is a probability based method of reasoning.  Where, unlike deduction, things are not 'necessary" but assigned varying degrees of certainty or likelihood. The advantage we have over deduction is that we can expand our knowledge base because we are moving often from specific cases to a general case.  If things seem to fit with our inferences they can be said to be "cogent" and assigned a likelihood (depending on the amount of data and strength of connections).  If things seem to not fit with inferences they can be said to be "not cogent".  These terms are the equivalent of sound and unsound in deduction.

Abduction: Basically this is telling a story that seems to fit with the facts. 

Here is an example of abduction.   

There is no pudding in the fridge.   We attempt to say why, by considering that someone ate all the pudding.  This seems like a reasonable explanation.  But first we should consider a few things. What other explanations are there, how about these two. The pudding was bad so someone threw it out; There was never pudding in the fridge.      Here we have three equally reasonable explanations, given the facts at hand. If we get more information then perhaps one or all of them will become less reasonable. For instance, if we ourselves put pudding in the fridge then we can dismiss #3 If we check the garbage and find the pudding #2 grows more likely.     If it is a new fridge still in the store we can bet that #3 is the most likely.

We could also postulate some form of pudding eating phantasm or perhaps suggest that Bill Cosby comes out of the television and eats it while we sleep.  The ability to tell a story the describes the events is simply the starting place.  How we attempt to support our claims is what really matters.

Now the state of pudding in the fridge is perhaps not your main concern (perhaps you've already had dessert).  But the methodology is one to be aware of. 

These three methods of reasoning are useful under differing circumstances and we should have at least a passing familiarity with all three. I am hoping that this basic guide will assist in differentiating the type of reasoning that is being used (if indeed reasons are being given). The evaluation of arguments is based partially on general knowledge and ability to apply these various forms at the appropriate times. 

Additional Reading

The Nature of Discourse and Argument series on this blog
Part 1
Part 2
Part 3
Part 4


  1. I think you abducted the pudding.

    1. That made my day. Now if only I had used an example with aliens instead.