Critical Thinking

Arguments can be strengthened or weakened by bringing in further evidence of a factual nature. 

Let's take a look at an example:

In Arthur Conan Doyle’s ‘The Adventure of the Norwood Builder’, Sherlock Holmes is dealing with what seems like a pretty cut and dry case. John McFarlane, a young lawyer, is accused of murdering one of his clients – a builder named Jonas Oldacre. He had a motive, was in the area at the time the murder was committed and perhaps most damningly, his stick was found at the scene of the crime. We might summarise this in argument form as follows:

• McFarlane stood to gain a considerable amount of money from Oldacre’s death (R1).
• He was in the area at the time the crime was committed (R2).
• His stick was found at the scene of the crime (evidence).
• Therefore, he is guilty of the murder of Jonas Oldacre (C).

Even Holmes’ is swayed by this reasoning but nevertheless has his doubts.  When a further piece of evidence is discovered:

• McFarlane’s bloody thumbprint (found on a wall in Oldacre’s house).

Lestrade, the police inspector investigating the crime, believes the case to be conclusive. However, Holmes, who had completed a thorough investigation of the house the day before and found no such thumbprint, is able to use this evidence to establish McFarlane’s innocence. He reasons as follows:

• There was no thumbprint on the wall in Oldacre’s house the day prior to its discovery (R1).
• McFarlane was locked up in gaol overnight (R2).
• Whoever placed the thumbprint there could not have been McFarlane (R3).
• So somebody is trying to frame McFarlane for murder (IC/R4).
• And this person, rather than McFarlane, must be the real culprit (MC).

SPOILER ALERT!!

With this additional piece of evidence, Holmes is able to flush out the real culprit (the very much alive Jonas Oldacre who had been running a revenge campaign against McFarlane!) and establish McFarlane’s innocence. 

Additional evidence, then, can be used to strengthen, weaken, confirm or refute an argument, hypothesis or explanation.  In order to test the effect that additional evidence might have on an argument, we need to ask ourselves the question:

‘if the following were true, would it:

a) strengthen/confirm,
b) weaken/refute, or
c) have no effect on the argument in question. 

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