The entire purpose of the logic we're looking at is assessing arguments. So it'll be helpful to understand exactly what an argument is. Here's a working definition:

ARGUMENT: A series of propositions consisting of premises which are purported to support a conclusion.

Of course, to understand this definition, we need to know what a proposition, premise, and conclusion are. A proposition is a special kind of statement - it's one that can be true or false. Obviously, questions like 'What time is it?' can't be true or false. And neither can commands like 'Shut the door.' A proposition says something about the world. Here are some examples of propositions:

1) It is sunny outside. 2) Mercury is the closest planet to the sun. 3) All mammals lay eggs.

Notice that 3 is false. But that's okay, it's still a proposition. Remember, these are statements that can be true OR false. As it turns out, there are some philosophers who have some strong arguments about what is and isn't a meaningful proposition. But that discussion is for an Analytic Philosophy class. We don't really care about these things in logic. If it's something to make sense to say it's true or false, then it's a proposition.

So an argument consists of premises and a conclusion. The premises are propositions that give you a reason to accept the conclusion, which is also a proposition. The conclusion is what you're supposed to, well, conclude! Here's as example:

1) All men are mortal. 2) Socrates is a man. 3) Therefore, Socrates is mortal.

In this argument, 1 and 2 are the premises which support 3, the conclusion. You can usually tell the conclusion by keywords like 'therefore' 'so' and 'thus.' Look back at the definition of an argument - notice it says that the premises are PURPORTED to support the conclusion. That just means that they are intended to give support - but they may fail miserably. The result would be a bad argument, but it's still an argument. Here's an example:

1) Monkeys like bananas. 2) I like bananas. 3) Therefore, I'm a monkey.

In this argument, the premises lend very little support to the conclusion. You may even have an argument where the premises have nothing at all to do with the conclusion. But these are still arguments - just really bad ones!

So now you know what an argument is. Up next, we'll go over the basics of how to assess an argument. This, remember, is the central goal of logic (at least, the logic we're talking about). It's worth noting here that the kind of logic we'll be talking about is called PROPOSITIONAL LOGIC. This logic deals with, you guessed it, propositions. Overall, it's very weak - there are many arguments it can't assess. More powerful logical systems like predicate logic and modal logic can handle more arguments. But you have to walk before you can run, and this kind of logic is a very good place to start. If you can understand this, you'll have a much easier time learning more powerful logical systems.

These are the basics, so if there are any questions, please post them. It's vital that you understand these definitions so that the next part will make sense.

Santi, that statement is solipsistic if I'm not wrong. Solipsism as an epistemological position holds that knowledge of anything outside one's own mind is unsure.

Either way, I would just doubt you, since I've seen you tell people that you twist the concept of logic, wrap it up in nice sentences and toss it at us whilst not even knowing what it means.

But this thread isn't really the place to debate logic itself. That's why I called it 'An Introduction to Logic' and not 'Post Your Idiotic Ideas About Logic Here'.

you r talking abt logic using ur logic to prove certain things abt logic? *.* everyone has different views..... mine is that you all are just arguing abt logic using ur logic here... but either way arguing can be gud :P so continue continue.... .:P n i dont understand many words here.... :/

Well, people can lack off their typing, and I don't mind. But, I can probably tell that some people hate it because maybe that "someone" cannot be able to read what you're saying.

I find that Aristotle explanation of logic is quite accurate and i recommend The Golden Rule by Aristotle it is a great mind teaser. He is one of the first people to want answers and conclusions and he asks why and dosent just say yes he says maybe then decides through diligent thought and scientific study. he is also one of the first people to ever classify animals by how they look.

Fruits are edible. Vegetables are edible. Therefore, fruits are vegetables.

There! That should solve all those conspiracies about, "Is pumpkin a fruit or a vegetable?"

So here we have a prime example of a logical fallacy. This fallacy is knows as 'affirming the consequent'. I haven't looked through the 'lecture' pages in quite some time, but I would hope that this fallacy should be evident through those discussions. So clearly the above argument isn't right. In other words, the conclusion is clearly false. So when we go about assessing an argument with an obviously false conclusion, we need to figure whether 1) the argument is invalid, or 2) the argument is unsound. For the most part, people will give argument that are unsound - that is, you can reject one of the premises as being false. But the argument we have on hand in invalid - i.e. not valid. Can you see why? Remember that validity is a feature of an argument's structure. It doesn't matter whether the premises are true - this is a matter of whether the argument has a valid structure or not. You can use logic to show that an argument is invalid (although the logic that is presented in this thread is not powerful enough to do this). But you can also use a more intuitive method. All you need to do is present an argument with the same structure that is clearly invalid.

So, here's the structure of the above argument:

All X are Z. All Y are Z. Therefore, all X are Y.

Now all you have to do is plug in terms to show that the above argument is clearly invalid (although the fact that the premises are true and the conclusion false should give this away immediately). But try this on:

All dogs are mammals. All cats are mammals. Therefore, all dogs are cats.

Again, we've just plugged different terms into the structure of the argument, and clearly this argument has a false conclusion. This is because the argument structure is invalid. As I mentioned before, this is the fallacy of affirming the consequent, and we can see now why this is a fallacy. It takes more sophisticated logic to prove this (compared to what's been presented in this thread) but it should be intuitively clear that this argument is invalid.a

After reading this, I feel as though I will win any debate/argument/court case. In the case of: All dogs are mammals. All cats are mammals. Therefore, all dogs are cats And assuming you dont know the conclusion is false, we must also assess the validity of the Subjects in each line. IE: Dogs, Cats, and mammals. By working each Subject down to atomic level and finding at least ONE difference between dogs and cats, we therefore prove that the conclusion is false. IE: counterexample. Wow, with logic, you really dont need to know anything in order to learn everything.