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Tutorial 3 - Multi-reason Arguments

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Multi-reason arguments

Sometimes there is more than one reason for a particular contention.  Such situations are called multi-reason arguments.

 
Discussion

Example:

The Moon rocks are unlike any rocks on Earth. To start, the Moon rocks are covered in 'zap pits'. Also, the Moon rocks are anhydrous. [based on 9.2-3]

This piece of reasoning contains two separate reasons for the same contention. That is, there are two separate simple arguments, but they have the same contention.


The two simple arguments mapped separately.

The two simple arguments mapped on one argument "tree".

The core idea behind multi-reason arguments is that there is a single claim (the contention) with at least two quite distinct pieces of evidence bearing upon it.

In mapping multi-reason arguments, there is no need to represent the contention twice; it is simpler to have the two reasons pointing at the same claim box. 

Complex argument structures, of which the above is the most elementary kind, are called argument trees because they have a root (the main contention) and branches.  Note that logicians tend to draw their trees upside down. 

Technical note: There is also a more technical definition of tree, in graph theory and computer science, and complex argument structures meet that definition also, but only at high level or coarse grain; when you introduce co-premises, the strict definition no longer applies.

 
New Concepts

A multi-reason argument is an argument with more than one reason or objection for a single contention.

Multiple Objections

Similarly, there can be more than one objection to a single contention.

 
Discussion

Example:

There was no dust on the footpads of the Lunar Module (LM), or blast crater in the dust under the LM. Therefore, the Apollo astronauts did not land on the moon. [Based on 4.0]

 

 

Since an objection is really just a special sort of reason (i.e., a reason to think the contention is NOT true), we tend to use the term multi-reason argument to cover arguments with multiple objections (as well as mixed cases - see next page).

Counter-arguments

No prizes for having already guessed that there can also be a reason and an objection bearing on the same contention.  The reason and objection are then countering each other; they are known as counter-arguments, relative to each other

 
Discussion

Example:

The Apollo rock samples are genuine Moon rock.  They appear similar to terrestrial rocks on the surface, but chemically they are unlike anything on Earth. [Based on 9.1-3]

To be very precise, a counter-argument is not the reason or the objection itself.  A counter-argument is an argument, and a simple argument is a complex thing made up of a reason with its contention, or an objection with its contention.  So counter-arguments include the contention; and counterarguments overlap in the sense that they have the same contention.  This is probably going to make more sense if you see it on a diagram:

Two arguments which counter each other (and thus are counter-arguments).  Note that each simple argument includes the contention.  Thus, the two simple arguments overlap; the same contention is part of both of them.
 
New Concepts

A counter-argument to a reason is an objection to that reason's contention, and vice versa.

Disputes

A dispute is where there are both reasons AND objections bearing upon a single contention.

 
Discussion

These are just some of the arguments raised in Apollo Moon Landings, concerning whether or not the Apollo astronauts landed on the Moon.  They illustrate a dispute: evidence presented for and against a particular contention.

Strictly speaking, to have a dispute, you only need at least one reason and at least one objection relating to a single contention.  Typically however there will be a number of each type. 

Within a dispute, a reason and an objection are counter-arguments.

A dispute is a simple form of disagreement.  It contrasts with genuine debates, which are more complicated and interesting. We will consider debates in Tutorial 6.

 
New Concepts

A dispute is an argument in which there are both reasons and objections bearing upon a single contention.

Stranded Co-premises

Another common mistake - the flip side of the previous one - is putting co-premises of a single reason into separate "reasons".  If you do this, the co-premises are stranded from each other; they are not diagrammed as belonging together.

 
 
Discussion

Consider this example from Apollo Moon Landings, which we already saw in Tutorial 2:

The Lunar Roving Vehicle, was 3.1 metres long by 2.3 metres wide, and 1.14 metres high. As the Lunar Module's descent stage was only 4.3 metres in diameter by 3.2 metres high, one might think this would not have left much room for the remaining equipment. [8.4]

Two separate claims are made as part of the evidence that the Lunar Roving Vehicle would not leave much room for other equipment.  But these are not two separate arguments.  Rather, they are two interlocking parts of one argument.


 
 

Notice that if you applied the Rabbit rule to either of the reasons in the first (two reason) argument map, you'd see right away that there was at least one co-premise, and that co-premise would have to involve one of the key items in the other reason.  More generally, applying the Rabbit and Holding Hands principles will quickly reveal that the premises belong together as parts of one argument rather than separate arguments.

 
New Concepts

Stranded co-premises: Two premises are stranded from each other when they in fact belong together as part of one reason but are diagrammed as belonging to separate reasons.

Strange Bedfellows

The biggest challenge in correctly mapping multi-reason arguments is telling whether you have one reason or two.  If you have two claims, are they both parts of one reason, or parts of two entirely distinct reasons?  

One common mistake is treating separate reasons as co-premises within one reason.  These premises don't belong together; they are "strange bedfellows."

 
Discussion

Example:

These premises don't belong in one reason.  They are two separate pieces of evidence for the contention.

Here, the two premises have been put in different reasons.  Co-premises have been added so that the reasons satisfy Rabbit and Holding Hands.

 
New Concepts

Strange bedfellows: Two premises are strange bedfellows if they are diagrammed as co-premises in one reason when in fact they belong to completely different reasons.

Summary

 
Key Points

 
New Concepts

A multi-reason argument is an argument with more than one reason or objection for a single contention.

A counter-argument to a reason is an objection to that reason's contention, and vice versa.

A dispute is an argument in which there are both reasons and objections bearing upon a single contention.

Strange bedfellows: Two premises are strange bedfellows if they are diagrammed as co-premises in one reason when in fact they belong to completely different reasons.

Stranded co-premises: Two premises are stranded from each other when they in fact belong together as part of one reason but are diagrammed as belonging to separate reasons.

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 Last updated 28-Aug-2007