flawed logic

[Observer]

Ideas come to you on the strangest moments
Recently I was changing tires when I thought

My spare tire fits my car
my spare tire also fits my wife's car
Hence I can safely assume that my regular tires will fit my wife's car

This is logic right

Here it comes...
My cat recently died
my grandmother died as well
hence I can safely assume my grandmother was a cat

This is logic r...
wait a sec...
my grandmother didn't have whiskers...

Enough foolishness...
In this example it is clear that my logic is flawed. I can imagine this isn't always the case but I can't give an example. I was wondering if there is a rule of thumb to analyse this? A sort of checklist to find out?

[Kikko]

Quote:My spare tire fits my car
my spare tire also fits my wife's car
Hence I can safely assume that my regular tires will fit my wife's car

This is logic right

Here it comes...
My cat recently died
my grandmother died as well
hence I can safely assume my grandmother was a cat

Hence you can safely assume that the tire is suitable to both, your wifes and your car.
Hence you can safely assume that death came to both, your grandmother and your cat

[Observer]

(31-12-2010 07:35 AM)Kikko Wrote:  

Quote:My spare tire fits my car
my spare tire also fits my wife's car
Hence I can safely assume that my regular tires will fit my wife's car

This is logic right

Here it comes...
My cat recently died
my grandmother died as well
hence I can safely assume my grandmother was a cat

Hence you can safely assume that the tire is suitable to both, your wifes and your car.
Hence you can safely assume that death came to both, your grandmother and your cat

Thats one...

I know that... In this case... But it might be not so clear in other cases...

[gaglamesh731]

Sorry about your cat and grandmother though

[Observer]

(31-12-2010 09:41 AM)gaglamesh731 Wrote:  Sorry about your cat and grandmother though

the cat was only for the sake of argument.
as for grandmother
In 2007 she passed away peacefully at the age of 91 in the presence of her son while I was holding her hand (literally) .
The circle was round!

[Observer]

I started reading the book mentioned here but so far I did not really find a clue. I'm not trough yet.

[Free_Thinker]

(31-12-2010 07:23 AM)The_observer Wrote:  Ideas come to you on the strangest moments
Recently I was changing tires when I thought

My spare tire fits my car
my spare tire also fits my wife's car
Hence I can safely assume that my regular tires will fit my wife's car

This is logic right

Your tire logic is flawed..

A spare tire is a somewhat generic size and intended to be used temporarily only. For example, I used to have a Pontiac Grand Prix (full size car) and my dad had a Pontiac minivan. They had the same spare tire, but the only thing the regular tires (rims actually) had in common were the 5-bolt pattern. Yes, technically you could probably FIT one of the regular wheels onto the other, but there is the difference of the wheel size (15 inch/16 inch) and the tires themselves have 3 main variables- width of the tire, side wall size (how tall the tire is), and the size of wheel that the tire will fit onto. These affect the intended performance and also mismatched tires could cause damage to the vehicle.

If you and your wife have the same car and it is the same year and model, then you can almost assume that the wheels are the same, but not always!

[Observer]

LOL!

I can easily see why the tire would fit (the cars are indeed from the same make, it is just a different model). You are merely pointing out arguments for/against the example.

What I am asking for is whether there is a rule of thump to avoid flaws like in the second example.

If you want you can apply the theory to the next example:

1:
The climate has changed over the past decades
The population density have risen over the past decades
Thus climate-change and population density are related

2:
The climate has changed over the past decades
There where less pirates on the seas in the past decades
Thus climate change and pirate-ism are closely related

(bear in mind that I hold not a real position on that matter for the moment)

[ashley.hunt60]

I think that you need to put the logical steps in terms on none, some, or all in a third step. Something like a diagram, because you are missing the logical step that says if an event is unique to one person/group, or if it encompasses several groups.

My cat recently died
(All thing dead are cats?)
my grandmother died as well
hence I can safely assume my grandmother was a cat

My spare tire fits my car
(my spare tire can only fit on one size of car?)
my spare tire also fits my wife's car
Hence I can safely assume that my regular tires will fit my wife's car

You're other proofs are missing that point that makes it into more than just correlation, but causation. BTW, I just came up with this after several days of pondering, so don't think that this is undoubtedly right.

[Sacrieur]

You're attempting a logical maneuver known as a counter-example. It is used to show a particular argument form is false by starting with a clearly false conclusion, and finding true premises to back it up.

Quote:My spare tire fits my car
my spare tire also fits my wife's car
Hence I can safely assume that my regular tires will fit my wife's car

You're riding on the assumption that your regular tires are the same as your spire tire. We can prove this wrong via the counter-example method:

A fits B.
A fits C.
Therefore, D fits C.

A brick fits in my car.
A brick fits in my house.
Therefore, a space-shuttle will fit in my house.

You can see how this is false. Unless you explicitly say that your car can only fit one type of tire, or that your spare is the same size as your regular tires, then the argument is fallacious. Some assumptions are justified, like assuming that the Atlantic Ocean is between Europe and Africa, and the Americas.

Let's try another:

Quote:The climate has changed over the past decades
The population density have risen over the past decades
Thus climate-change and population density are related

A has C.
B has C.
Thus, A is related to B.

I feel as though you can see how this is fallacious without a counter example. Don't ride on so many assumptions.