Thinking Skills: Critical Thinking and Problem Solving

262 Unit 7 Critical reasoning: Advanced Level

Consider the following simple demonstration.

(You probably saw it at elementary school.) A

candle is placed in a shallow dish of water and

lit. A jar is then held over the candle so that

its rim is underwater to seal it from the air.

After a short while, the candle flame dies, and

some of the water rises inside the jar. The

procedure is exactly repeated three or four

times to demonstrate that it wasn’t a fluke.

The reason why the candle goes out, in

non-technical language, is that the flame burns

up the oxygen in the jar, and without oxygen

it can no longer burn.

You don’t need an argument to persuade

you that if you repeat the experiment a fifth or

7.3 Non-deductive reasoning

sixth time, the candle will go out. But, if you

were asked to spell out the argument, it might

go something like this:

[1] Every time a lighted candle is placed in a

sealed and restricted space (such as a

jar) it has been observed to go out

shortly afterwards. Therefore we can

infer that it always will.

Activity

Is this a valid form of argument? Is [1] a good

argument?

Commentary and continuation

The answer to the first question is no. It is not

valid. [1] is an example of a fallacy that is

sometimes called ‘appealing to history’. It is

claiming that because something has been

observed to be the case in the past, it will

always be so in the future. We can assume that

the single premise in [1] is true. It is based on

direct evidence, verified by a great many

experiments and demonstrations, none of

which has ever been observed to have a

different outcome. But the inference that this

will always be the case cannot be verified by

direct evidence. Therefore the premise could

be true and the conclusion false (under some

freak circumstance).

You could argue that the conclusion of [1]

was a practical certainty. The laws of physics

would have to change to make it false. But

logically it is still an uncertainty. Its truth may

be beyond reasonable doubt, in the world as we

know it, but it is not beyond all doubt, in all