02b. How to Argue - Philosophical Reasoning. Part 2/2.
So, here's a boiled-down version of a good deductive argument:
● Premise 1: All humans are mortal.
● Premise 2: Socrates is a human.
● Conclusion: Socrates is mortal.
This kind of reasoning, where one fact leads to another, is called entailment.
Once we know that all humans are mortal, and that Socrates is a human, those facts entail that Socrates is mortal. Deduction begins with the general – in this case, what we know about human mortality – and reasons down to the specific – Socrates in particular.
What's great about deductive arguments is that the truth of the premises must lead to the truth of the conclusion.
When this happens, we say that the argument is valid – there's just no way for the conclusion to be false if the premises are true.
Now check out this argument: All humans are mortal. Socrates is a human.
Therefore, Socrates was Plato's teacher.
That argument is invalid, because nothing about human mortality can prove that Socrates was Plato's teacher. As you might have noticed, there are plenty of mortal humans who never taught Plato.
What's interesting, though, is that this argument does happen to have a true conclusion, which leads us to another issue. And that is: Validity is not the same as truth.
All ‘valid' really means is that if the premises are true, then your conclusion can't be false. But that doesn't mean that your premises prove your conclusion to be correct. Like, in the case of whether Socrates was Plato's teacher, the premises are true, and the conclusion is true, but the argument is still not valid -- because the premises don't in any way prove the conclusion. It just happens to be true.
So, if your premises don't guarantee the truth of your conclusion, then you can end up with some really crappy arguments. Like this one:
- All cats are mammals
- I'm a mammal
- Therefore, I'm a cat
As much as part of me would like to be my cat, this is invalid because the conclusion doesn't entail from the premises…at all.
I mean, all cats are mammals, but all mammals aren't cats. Which means there are such things as non-cat mammals, which I am just one example of. And it probably goes without saying, but you can have a perfectly valid argument and still have a false conclusion, if any of your premises are false. For example:
- All humans have tails
- My brother John is a human
- Therefore, John Green has a tail!
The argument is totally valid! – Because the premises entail the conclusion! The reasoning totally stands up! It's just that one of the premises is flawed.
Since I'm reasonably certain that John doesn't have a tail -- I've seen him in a bathing suit -- this argument is not deductively sound. And a deductively sound argument is one that's free of formal flaws or defects.
It's an argument whose premises are all true, and that's valid, which means its conclusion is guaranteed to be true.
So, sound arguments should always be your goal. The reason that deduction is prized by philosophers -- and lots of other important kinds of thinkers -- is that it's the only kind of argument that can give you a real certainty.
But it's limited, because it only works if you're starting with known, true premises, which are hard to come by. And for what it's worth, deductive truths are usually pretty obvious. They don't tend to lead us to startlingly new information, like the fact that I'm not a cat, or that John doesn't have a tail.
So instead of starting with premises that are already certain, like deduction does, you're gonna have to know how to determine the truth of, and your confidence in, your premises. Which means you're going to have to acquaint yourself with the other species of arguments, which we're gonna do next time.
But today, we talked about the value of reason, the structure of arguments, and we took a close look at one kind of argument: deductive reasoning.
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