WELCOME TO AS PHILOSOPHY!
Please read the article below and then complete Tasks 1, 2 and
3 on the following pages.
Please bring your answers to your first Philosophy lesson.
We look forward to meeting you soon.
Sabina, Rekha, Louise and Jane
Deductive and Inductive arguments
Valid arguments are known as DEDUCTIVE or ‘deductively valid’. In a deductive argument the
truth of the premises guarantees the truth of the conclusion, so long as no errors have been made.
The key word here is ‘guarantee’. With a deductive argument, if we accept the premises to be
true then we absolutely must accept the conclusion to be true. If, as we’ve claimed, the goal of an
argument is to persuade people to believe its conclusion, then deductive arguments must be a
powerful tool: after all, if we can guarantee the truth of a conclusion, we have good reason to
believe it. However, this great strength can also appear as a weakness. For deductive arguments
can’t establish anything new with their conclusions: they simply reveal what is already contained
in the premises. For this reason, they don’t really get us beyond what is already known. Another
weakness is that, whilst we can know that the conclusion must follow if the premises are true, we
still can’t guarantee that the premises actually are true. Knowing that the conclusion has to follow
from the premises is all very well, but it simply passes the buck and we still have to find a way to
establish the truth of the premises. To make clearer the strengths and weakness of such
arguments, take the following standard example of a deductively valid argument:
1. All men are mortal.
2. Socrates is a man
3. (Conclusion) Therefore Socrates is mortal.
Here, if we accept the two premises, then the conclusion follows necessarily. The great strength
of the argument is that it appears to be impossible to deny the conclusion once we’ve accepted
that the premises are true. We might say that so long as we accept the premises to be true then
we can work out the truth of this conclusion in an A PRIORI manner, in other words prior to any
further experience or fact-finding, simply by teasing out what is implicit in what is already given in
the premises. Everything can be done in our heads. However, this also means that we haven’t
really learned anything new here. The conclusion says nothing more than was already contained
in the premises. Moreover, whilst we know that Socrates must be mortal if he is a man, and if all
men are mortal, we have still to find a way of establishing that these other facts are actually true.
So deductive arguments appear to leave us with further questions to address, namely how to
establish the truth of the premises.
INDUCTIVE ARGUMENTS are often contrasted with deductive ones because they strive to reveal
something new in their conclusion. This is one of the strengths of such arguments. However,
because this means they have to go beyond the information contained in their premises, they lose
the power to guarantee the truth of their conclusions. This means that no inductive argument can
be fully valid, and at best their conclusions are only ever probably true, even if the premises are
certainly true. Typically, induction occurs where an argument moves from what is known (e.g.
facts about the past, or particular observations) to what is unknown (e.g. speculations about the
future, or generalisations). Induction is frequently used in the sciences and social sciences,
whenever we move from empirical data to theories about the data. A typical inductive argument
might be:
1. Every raven I’ve ever seen has been black.
2. There are 100 ravens kept at the Tower of London.
3. (Conclusion) Therefore it is likely that all 100 ravens in the Tower are black.
Even if we accept the premises as definitely true, this conclusion doesn’t necessarily follow. This is
because there might be whole families of London-born white ravens, which I don’t know about, or
it might be that for every 100 black ravens in a population two of them will be white. Just because
I have only ever seen black ravens doesn’t establish that they must all be black. Note that, just as
with deductive arguments, we still have to accept the premises as true before the argument can
be at all convincing.
Another example of an inductive argument is one where we draw a conclusion from a finite set of
instances:
1. John ‘Stumpy’ Pepys used to play drums for rock ‘n’ roll legends Spinal Tap but he died in
mysterious circumstances.
2. Eric ‘Stumpy Joe’ Childs used to play drums for Spinal Tap but he too died in mysterious
circumstances.
3. Peter ‘James’ Bond used to play drums for Spinal Tap but he eventually died in mysterious
circumstances.
4. Mick Shrimpton now plays drums for Spinal Tap.
5. (Conclusion) So it is likely that Mick Shrimpton will one day die in mysterious circumstances.
Here we have observed that these drummers are all similar to each other in one respect (they
were members of Spinal Tap), and concluded that the new drummer must be similar to the others
in some further respect (he will die in mysterious circumstances). Of course, the conclusion does
not follow necessarily from the premises. Mick Shrimpton may be different from the others; he
may evade the Grim Reaper of Rock and live to a ripe old age. So, like other inductive arguments,
this one cannot establish its conclusion with absolute certainty. Here the strength of the
argument will depend on how strong the similarities are between each case of drumming tragedy.
TASK 1: In your own words, outline the strengths and weaknesses of inductive and deductive
arguments.
1. Strengths of deductive argument
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2. Weaknesses of deductive argument
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3. Strengths of inductive argument
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4. Weaknesses of inductive argument
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TASK 2: Read through the following arguments and, for each argument, answer the following
questions:
a) What is the conclusion of each one?
b) Which are inductive arguments, and which are deductive?
c) Which arguments do you think work and which don’t? Why?
1. I’ve split up with every person I’ve ever been out with, so the relationship I’m in at the
moment is bound to end too.
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2. Men are incapable of driving safely, because they are prone to uncontrollable hormonal
changes that can lead to road rage.
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3. If you don’t believe in God you will go to hell. Stuart doesn’t believe in God. So Stuart will go
to hell.
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4. Philosophers spend much of their time sitting around and thinking, which means their muscles
weaken, and so none of them is any good at strenuous exercise such as lifting weights.
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5. Paul says he had a vision of the Virgin Mary in his bedroom last night. Paul is known to be a
trustworthy person and so it is likely that the Virgin really did appear to him.
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6. It’s wrong to kill innocent people. But babies are people too, even when they’re in the womb.
So it’s wrong to have an abortion.
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7. The evil dictator denies he has any weapons of mass destruction. But we know from his
denials in the past that he’s a liar. So he must have them somewhere, and we should keep
looking until we find them.
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8. My little sister is three and she loves the adverts for toys, so your three-year-old sister should
enjoy them too.
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9. If God exists then he would have created a world without suffering and evil. However,
examples abound of terrible suffering and evil in the world. So God cannot exist.
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10. No England football team for the last 40 years has got through to the final of a major
competition. So they will obviously fail in the next World Cup.
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TASK 3: Extension
Can you think of your own examples of:
a) a deductive argument which works;
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b) a deductive argument which does not work;
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c) an inductive argument which works
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d) an inductive argument which does not work.
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