But, Semmelweis observed, the First and Second Clinics were very close to each other, had similar ventilation, and shared a common anteroom. So, they had similar atmospheric conditions. He reasoned: If childbed fever is caused by cosmic-atmospheric-terrestrial influences, then the mortality rate would be similar in the First and Second Clinics.
But the mortality rate was not similar in the First and Second Clinics. So, the childbed fever was not caused by cosmic-atmospheric-terrestrial influences.
Another hypothesis was that overcrowding caused the childbed fever. But, if overcrowding caused the childbed fever, then the more crowded of the two clinics should have the higher mortality rate. But, the Second Clinic was more crowded in part because, aware of its lower mortality rate, mothers fought desperately to be put there instead of in the First Clinic.
It did not have a higher mortality rate. So, the childbed fever was not caused by overcrowding. Another hypothesis was that fear caused the childbed fever. For reasons of the layout of the rooms, the priest delivering last rites in the First Clinic walked by all the rooms, ringing a bell announcing his approach.
This frightened patients; they could not tell if the priest was coming for them. Semmelweis arranged a different route for the priest and asked him to silence his bell. But it was not the case that the rate of childbed fever declined when people could not tell if the priest was coming to the First Clinic.
In the First Clinic, male doctors were trained; this was not true in the Second Clinic. These male doctors performed autopsies across the hall from the clinic, before delivering babies. Semmelweis knew of a doctor who cut himself while performing an autopsy, and who then died a terrible death not unlike that of the mothers who died of childbed fever. Semmelweis formed a hypothesis. The childbed fever was caused by something on the hands of the doctors, something that they picked up from corpses during autopsies, but that infected the women and infants.
He reasoned that: if the fever was caused by cadaveric matter on the hands of the doctors, then the mortality rate would drop when doctors washed their hands with chlorinated water before delivering babies.
He forced the doctors to do this. The result was that the mortality rate dropped to a rate below that even of the Second Clinic. He was the first person to see that washing of hands with sterilizing cleaners would save thousands of lives. It is hard to overstate how important this contribution is to human well being. But how can we be sure his reasoning was good?
Semmelweis was essentially considering a series of arguments. Let us turn to the question: how shall we evaluate arguments? Our logical language now allows us to say conditional and negation statements. That may not seem like much, but our language is now complex enough for us to develop the idea of using our logic not just to describe things, but also to reason about those things. We will think of reasoning as providing an argument. In such a case, someone is using language to try to convince us that something is true.
Our goal is to make this notion very precise, and then identify what makes an argument good. We need to begin by making the notion of an argument precise.
Our logical language so far contains only sentences. An argument will, therefore, consist of sentences. However, we want to separate the notion of a good argument from the notion of an argument, so we can identify what makes an argument good, and what makes an argument bad. To do this, we will start with a minimal notion of what an argument is. Here is the simplest, most minimal notion:. This is obviously very weak. There is a famous Monty Python skit where one of the comedians ridicules the very idea that such a thing could be called an argument.
But for our purposes, this is a useful notion because it is very clearly defined, and we can now ask, what makes an argument good? The everyday notion of an argument is that it is used to convince us to believe something. The thing that we are being encouraged to believe is the conclusion.
But belief is a psychological notion. We instead are interested only in truth. So, we can reformulate this intuitive notion of what an argument should do, and think of an argument as being used to show that something is true.
The premises of the argument are meant to show us that the conclusion is true. What then should be this relation between the premises and the conclusion? Intuitive notions include that the premises should support the conclusion, or corroborate the conclusion, or make the conclusion true. What we can use in their place is a stronger standard: let us say as a first approximation that if the premises are true, the conclusion is true. But even this seems weak, on reflection.
For, the conclusion could be true by accident, for reasons unrelated to our premises. Remember that we define the conditional as true if the antecedent and consequent are true. But this could happen by accident. Suppose also that Tom both wears blue and Tom gets an A on the exam. This makes the conditional true, but we hope the color of his clothes really had nothing to do with his performance on the exam. A better and stronger standard would be that, necessarily, given true premises, the conclusion is true.
This points us to our definition of a good argument. Valid argument: an argument for which, necessarily, if the premises are true, then the conclusion is true. A bad argument is an argument that is not valid. We cannot make rules of logical reasoning until we know what we want those rules to do, and what we want them to do is to create valid arguments.
This is not required. There are valid arguments with false premises and a false conclusion. If A happens, then B happens too. This structure is called the major premise of the argument.
This argument form is known as modus ponens. The ground is wet. Therefore, it is raining. At first glance, one may be tempted to believe that this argument shows good reasoning as well. Since we have already established the truth-value of the first premise, and since we will assume that somewhere the ground is, in fact, wet, let us consider the relationship between the premises and the conclusion.
This argument is invalid: the truth of the premises does not give us grounds to accept the conclusion as true. There are reasons other than rain that the ground could be wet: the sprinkler system could be on, for instance. Falsely inferring that it is raining because the ground is wet is known as affirming the consequent. This fallacy and others are discussed in greater detail in our logical flaw sections: general inferential errors , errors in causality , errors in generalization , and other fallacies.
The First Characteristic: True Premises. This first condition for a cogent argument is the one likely to involve the most research. An important decision writers must make in their papers is when to take a premise as accepted or whether it needs support from outside research.
Judgment in this regard is often formed with experience writing and familiarity with the topic. In any case, many premises will have to be supported in some fashion. See our page on strategies for reasoning and avoiding fallacies for more advice on this topic. It is also important to identify to your audience that a statement is a premise of your argument.
A conclusion often takes the form of a topic sentence; thus, your reasons to accept this conclusion may fall under the umbrella of your topic sentence. You can unlock new opportunities with unlimited access to hundreds of online short courses for a year by subscribing to our Unlimited package. Build your knowledge with top universities and organisations. Learn more about how FutureLearn is transforming access to education.
Learn more about this course. Good and bad arguments Good arguments are either cogent or sound, otherwise they are bad. Loved It 05 Jul, The contents were worth the time and I Visit the course. Excelent course regarding making Hand on Course 01 Jul, I Have just taken a short course on the same topic in Chile and I think this one was the perfect decision to clarify doubts and put knowledge into Thanks to the wonderful lecturers and the great community of Good course 01 Jul, Excellent course, move at your o It is better than I expected 14 Sep, Easy to understa Thanks for this wonderful course.
I enjoyed it very much and I learned a lot too. Best regards,. We now have all the ingredients to be able to tell when arguments are good or bad. Want to keep learning? This content is taken from The University of Auckland online course,. If there is a purple elephant in the hall, then I am a giant turkey. Then the argument is sound, and is, therefore, good. It's too vague.
We have to say more about what we mean by "good reasons" obviously. To do this we'll start by looking at some ways that arguments can fail to be good, and from these we'll extract a couple of necessary conditions for an argument to be good.
All actors are robots. Tom Cruise is an actor. Therefore, Tom Cruise is a robot. All tigers are mammals. Tony is a mammal. Therefore, Tony is a tiger. Both of these are bad arguments, as you might be able to see. But they're bad in different ways. In the first argument on top, the problem is obviously that the first premise is false — all actors are not robots.
But that's the only problem with this argument. In particular, the logic of this argument is perfectly good. When I say that the logic is good what I mean is that the premises logically support or imply the conclusion, or that the conclusion follows from the premises.
In this case it's clear that if the premises of this argument were all true then the conclusion would have to be true, right? If all actors were robots, and if Tom Cruise is an actor, then it would follow that Tom Cruise would have to be a robot.
The conclusion does follow from these premises. Are all tigers mammals? Yes they are. Is Tony a mammal? Well in this case we have no reason to question the premise so we can stipulate that it's true.
So in this argument all the premises are true. This is where we have a problem. Just because all tigers are mammals and Tony is a mammal, it doesn't follow that Tony has to be a tiger. Tony could be a dog or a cat or a mouse. So even if we grant that all the premises are true, those premises don't give us good reason to accept that conclusion.
So this argument has the opposite problem of the first argument.
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