Posted by alexandra_k on May 16, 2005, at 18:35:57
In reply to Re: I'm afraid » alexandra_k, posted by Damos on May 16, 2005, at 18:08:48
This might just put you off...
If it doesn't make TOOOO much sense - don't worry.
Just have a go at the questions
(If you like)An argument is an attempt to get you to do or believe something by providing reasons.
The following are propositions (claims), but they are not arguments because they do not provide reasons:
It is raining
It is wetPropositions can be used as parts of arguments:
(a)
If it is raining then it is wet. It is raining. Therefore, it is wet.(b)
If it is raining then it is wet. It is wet. Therefore, it is raining.(At this stage it looks a little silly – but it pays not to complicate things unnecessarily.)
The conclusion of the argument is what the argument is attempting to persuade you of. The reasons given in support of the conclusion are the premises.
In order to make the structure of the argument clearer we can set the argument out in standard form as follows:
(a)
(P1) If it is raining then it is wet.
(P2) It is raining.
______________________
(C) It is wet.(b)
(P1) If it is raining then it is wet.
(P2) It is wet.
___________________________
(C) It is raining.
(P1), (P2)… Stand for Premise 1, Premise 2... An argument can have one or more premise / premises. The line is called an ‘inference bar’. That shows you that everything above the line is intended to be reason to believe what is below the line. (C) stands for Conclusion.The first step in evaluating these arguments (to see whether we have rational grounds to accept the conclusion or not – to see whether they are good arguments) is to see whether they are VALID or INVALID.
To say that an argument is valid is to say that it is impossible for the premises to be true and the conclusion false. In other words, if the premises were true then the conclusion would have to be true – it could not be false.
NOTE: It is not to say that the premises ARE true – it is just to say that if we ASSUME that they are true then the truth of the conclusion is guaranteed.
(That is a tricky notion to try and explain / make sense of. It is much easier to judge whether an argument is valid or invalid – in fact you are born with an innate understanding of that. It is just much harder to come to understand what makes an argument valid or invalid.)
Lets look at (a).
(P1) tells us that if it were true that it was raining then it would just have to be the case that it is wet as well.
(P2) tells us that it is indeed true that it is raining.
(C) tells us that it follows from the ASSUMED truth of both (P1) and (P2) that it JUST HAS TO BE wet.Suppose that the conclusion was false (suppose it is not wet). Then it just could not be the case that the premises are both true – without contradiction. If the premises are both true and the conclusion false then the conclusion would contradict the premises – which is not allowed in logic. This argument is therefore, VALID.
Lets look at (b).
(P1) tells us that if it were true that it was raining then it would just have to be the case that it is wet as well.
(P2) tells us that it is indeed true that it is wet.
(C) tells us that it follows from the ASSUMED truth of both (P1) and (P2) that it JUST HAS TO BE raining.Suppose that the conclusion was false (suppose it is not raining). This can be the case even while we are assuming that the premises are both true. That does not result in a contradiction. I could have turned on the hose and it could be wet in virtue of that. It does not have to be raining just because it is wet. The assumed truth of the premises does not rule that scenario out and so we can describe a case where the premises are true and the conclusion false without contradiction. The argument is therefore, INVALID.
So validity does not tell us whether the premises or the conclusion actually are true or not. It just tells us that the arguments structure is such that if the premises did happen to be true then the truth of the conclusion is guaranteed. Validity is a crucial (dare I say THE crucial) notion in logic. Logic thus shows us WHAT follows from WHAT. If we assume the world to be a certain way (the way that the premises describe) then logic shows us what follows from that (ie the conclusion). But logic does not tell us what IS or IS NOT the case in the world to start with (whether the premises are in fact true or false). The truth of the premises must be assumed. (That is the price of certainty peoples).
If an argument is invalid then we don’t even need to bother to assess whether the premises describe the world accurately or not. Even if the premises are true they do not guarantee the truth of the conclusion.
(Just because an argument is invalid doesn’t necessarily make it a bad argument. Inductive arguments don’t guarantee the truth of the conclusion, but they should (when all goes well) make the conclusion probable. Still… Best to leave them aside for the moment).
Are the following arguments valid or invalid?
(c)
(P1) If it is snowing then I am cold
(P2) I am cold
____________________
(C) It is snowing(d)
(P1) If it is snowing then I am cold
(P2) It is snowing
___________________________
(C) I am coldOr are peoples very lost???
poster:alexandra_k
thread:498245
URL: http://www.dr-bob.org/babble/social/20050513/msgs/498646.html