It’s not working for me on his argument.
It’s not working for me on his argument.
Just a thought. I may not be raising a valid point......
Is it confusing that argument 3 (above) and indeed the OP argument, are using categories (hope that's the right word) that are, in the real world, mutually exclusive (Joe can't in the real world be a squid and an elephant, given the definitions of what those are)?
Does it help if we use categories that aren't mutually exclusive?
P1: No tall person is a fat person.
P2: Joe is either a black person or a tall person.
P3: Joe is a fat person.
C: Therefore, Joe is a black person.
Or am I talking shite?
All I can say is that it is easier for me to see how that's valid, I guess.
I think there’s advantages and disadvantages. One advantage is that we can more easily relate and perhaps better think it through if we do as you say; however, truth (or even soundness) is often a distraction when trying to hone in specifically on validity. Another option is to use letters, but it’s easier to use words when trying to recollect how each part fits. At any rate, just try not to let the labels (that do refer to categories) get to ya.
It's not, in the context of these arguments, since it stands for any proposition you want.Originally Posted by fast
Excellent!
So, from the premises P1: No giraffe is an elephant, P2: Joe is either a squid or a giraffe, and P3: Joe is an elephant, you can validly conclude that Joe is a squid. Great!
Now, take a look at the argument in the OP. It turns out it contains all of those three premises!. Now, it also contains more premises, but even without those extra premises (and without the contradiction stuff), just using P1, P2 and P3, you can conclude validly that Joe is a squid. You should change your vote to "valid".
(or do you think that it's possible that from some premises P1,..,Pn you validly deduce Q, but if you add more premises, then you no longer can validly deduce Q?)
Let me try to persuade you otherwise.
Argument 3:
P1: No giraffe is an elephant.
P2: Joe is either a squid or a giraffe.
P3: Joe is an elephant.
C: Therefore, Joe is a squid.
Is that valid?
Consider, for example, the following argument:
Argument 4:
P1: No giraffe is a mollusc.
P2: Joe is either a squid or a giraffe.
P3: Joe is an mollusc.
C: Therefore, Joe is a squid.
If you take a look, you will see that Argument 4 and Argument 3 have exactly the same form. Before I go on, I would like to ask you whether you think Argument 3 and Argument 4 are both valid, or neither, or only one (depending on the answer, I might or might not try to convince you that the original OP argument is valid).
No squid is a giraffe
No giraffe is an elephant
No elephant is a squid
Joe is either a squid or a giraffe
Joe is an elephant
Therefore, Joe is a squid
Validity is about finding a path to the finish line where the conclusion is the final destination.
Each premise is like a path. Consider all the paths. Each premise bolded above (if followed) will lead us to the end.
Of course it’s flawed. It’s unsound. That is the great warning sign we should look out for with deductive arguments (whether it’s sound or unsound,) but there are some other things about arguments we can point out. For instance, whether a proposition (either in a premise or conclusion) is true. Another thing we can look at is validity. What that is exactly is in dispute. Do we go by dictionaries that explain how it’s ordinarily used, or do we use it as a technical term? When used as a technical term, we need to be careful about what it is and what it isn’t. We can have a perfectly valid argument and it still be unsound.
A) If Kansas is in the ocean, then Kansas is a squirrel.
B) Kansas is in the ocean.
Therefore C) Kansas is a squirrel
That’s valid because those two taken together leads to the conclusion.
A) If Kansas is in the ocean, then Kansas is a squirrel.
B) Kansas is in the ocean.
C) Bobby has an upset stomach
Therefore D) Kansas is a squirrel
That’s still valid. It’s like a math problem where information that is given is irrelevant. Just ignore it.
A) If Kansas is in the ocean, then Kansas is a squirrel.
B) Kansas is in the ocean.
C) Bobby has an upset stomach
D) Kansas is in space
Therefore E) Kansas is a squirrel
Obviously, if Kansas is in the ocean, then it’s not in space, but forget that. Just look to see there’s enough of the right premises to get to the conclusion —ignoring all others that merely serve to distract. A and B gets us to E, so it’s valid.
Now here’s a fun one:
A) Bobby has an upset stomach
B) bobby doesn’t have an upset stomach
Therefore C) Kansas is a squirrel.
That too is valid. Gotta love it.
You might be wondering what kind of logic allows that, as it might not always accord with what a dictionary has to say. But, it’s in the tool belt of the logician and well respected the world over. It just takes a little getting used to.
Gotta say it. The panda in the room. Its actually an Octopussy.