Since evidence is ultimately a set of reasoning(s) that justifies a claim/conclusion (see the previous two blogposts), the strongest evidence would therefore be the strongest kind of reasoning. A question then arises: what is the strongest kind of reasoning?
There are two kinds of reasoning in terms of how strong the reasoning justifies a claim or conclusion:
(1) Non-deductive Reasoning
This kind of reasoning, when done correctly, will always produce a conclusion that is UNLIKELY TO BE FALSE, from one or more pieces of true information. In other words, as long as true information/premises are being used, a properly executed non-deductive reasoning will always produce a conclusion that is probably true (there remains a small probability that the conclusion is false).
Non-deductive reasoning is divided into abductive reasoning (also known as “inference to the best explanation”) and inductive reasoning.
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An example of abductive reasoning (ie inference to the best explanation):
Information/Premise: While strolling in his garden, James was surprised to see a group of small pebbles positioned on the grass in such a manner that their overall pattern resembles a picture of Pope Francis’ face.
Conclusion/Claim: Someone has arranged the small pebbles that way. (this is the best natural explanation for the above pattern)
The above conclusion is unlikely to be false as long as the given information/premise is true.
(Nevertheless, the conclusion is still possible to be false. There is an extremely small probability that wind and/or other elements of nature have caused those pebbles to be coincidentally positioned in that way. Such a possibility cannot be totally ruled out.)
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An example of inductive reasoning:
Information/Premise: All the swans seen by humans over many hundreds of years were either white or black in colour.
Conclusion/Claim: Every swan is either white or black in colour.
The above conclusion is unlikely to be false as long as the given information/premise is true.
(Nevertheless, the conclusion is still possible to be false. For example, there is an extremely small probability that natural rainbow-coloured swans exist somewhere in nature, yet to be discovered by humans. The possibility of undiscovered natural rainbow-coloured swans cannot be totally ruled out.)
(2) Deductive Reasoning
This kind of reasoning, when done correctly, will always produce a conclusion that is IMPOSSIBLE TO BE FALSE from one or more pieces of true information. In other words, as long as true information/premises are being used, a properly executed deductive reasoning will always produce a conclusion that is certainly true.
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An example of deductive reasoning:
Information/Premise 1: James is taller than John.
Information/Premise 2: Mary is not taller than John.
Conclusion/Claim: James is taller than Mary.
Notice that as long as all the information/premises are true, it is impossible for the above conclusion/claim to be false.
Strongest Kind of Reasoning
From the above, we should see that the strongest kind of reasoning would be deductive reasoning. This is because deductive reasoning, when done correctly, always produces conclusions that are IMPOSSIBLE to be false, as long as true information/premises are used. In contrast, non-deductive reasonings (abductive and inductive reasonings) can only produce conclusions that are UNLIKELY to be false.
The only way for a deductive conclusion/claim (ie a conclusion/claim justified by deductive reasoning) to be false is when
(a) one or more of the premises/information used is false,
or
(b) the reasoning structure/form used is actually not a valid deductive structure/form (ie the reasoning is done wrongly and therefore it is not a true deductive reasoning in the first place).
As long as the deductive reasoning is done correctly (ie it has a valid deductive structure/form) and the information/premises used are all true, the conclusion/claim would be IMPOSSIBLE TO BE FALSE (ie deductively sound).