GENERALIZACIÓN DE LA LEY DE IGUALACIÓN A LAS SITUACIONES DE ELECCIÓN FORZADA

The verb ‘infer’ means to draw a conclusion, usually from some factual (or supposedly factual) information. The information provides the grounds for the inference. If the grounds are good, we say that the inference is sound, or reliable. Another word that is often used is ‘safe’. If the grounds for an inference are poor, we have to say that the inference is unsafe: it cannot be relied upon.

Judging whether an inference is safe is therefore similar to judging whether an argument is sound. The main difference is that in a standard argument the conclusion is stated. In many texts and documents,

however, there is no explicit conclusion. There may be claims; there may be information. But unless some further inference is drawn from the information, there is no argument. Sometimes there may be an implicit

conclusion, where the information is clearly leading in one particular direction, and it is obvious what we are meant to infer. There is an example in Chapter 2.4, item [7] (page 34). Here is another example (the subject may sound familiar):

[1] These banknotes all have the same serial number. All genuine banknotes have different numbers.

This time the conclusion has been left unsaid. But it is clear what the author is getting at. If asked what can be inferred from [1] most people would probably answer that:

[2] The banknotes are not all genuine. Moreover this would be a safe inference, because [1] provides very good grounds for [2].

Of course [2] is not the only inference that could be drawn from [1]. Nor is it the only safe one. [1] also gives good grounds for inferring

that I would be committing an offence if I tried to spend these banknotes. I would need to know that passing false currency is illegal, but that is such common knowledge that it practically goes without saying.

It would be fairly safe, too, to infer from [1] that

[3] The banknotes are forgeries, on the reasonable assumption that only forgeries could have duplicate numbers. By contrast it would be entirely unsafe to infer that

[4] The banknotes are the work of terrorists, intent on destabilising the economy. Inferring [4] from [1] would be a blatant example of jumping to a conclusion.

Inference and science

Drawing inferences, and judging the reliability of inferences, are especially important in scientific contexts. Scientists typically base their claims to knowledge on the information they collect from observation and experiment. Because we cannot confirm the truth of such claims without supporting evidence, we have at least to be sure that the evidence is strong and the inference is reliable. Assessing what can and cannot be inferred from a given document is therefore a key component of critical thinking.

Here is a short introductory example:

DOC 1

Ice ages last for roughly 100,000 years, going by the record of the past half-million years. The warm phases in between are called interglacials. The standard view, until quite recently, has been that we are

Commentary

We’ll consider the inferences in turn. The first one, A, as well as being a bit vague, has little support from the text. If we take ‘any

millennium now’ to mean in the next one or two – which is its natural meaning – then A would mean sticking with the standard view, despite the most recent findings casting doubt upon it. The standard view is that the present warm phase should be reaching its time limit; but according to the last three warm phases the limit has already been passed. The latest ice core also suggests that some interglacials could last as long as 30,000 years. Evidence that the Earth’s present alignment with the Sun’s rays resembles that of the last long interglacial, pours even more cold water on A. (Interestingly, even without the most recent evidence, the grounds for A would still be weak: half-a- million years is a blink of an eye in geological terms, and the sample of just three warm phases is too small to call a reliable trend.)

B might seem consistent with what has just been said about A. If we are not near the start of a new ice age, the standard view must be wrong. But there is a lot of difference between saying that an inference is unsafe, and declaring it false. B is a much stronger claim than can be supported by the data in Doc 1. We might be near the end of a warm phase: one that is longer than the last three and shorter than the one before that. There is little or no positive evidence for such a claim; but nor is there proof that it is false. Remember the significance of strong and weak claims (see Chapter 2.2). A strong claim requires much more to justify it than a more moderate claim. Had B asserted that the standard view is now less plausible than it was, instead of plainly false, that would have been defensible.

C suffers from the same fault as B: it, also, is too strong. A single example of a 30,000-year coming to the end of the present warm

phase, which has already lasted just over 10,000 years. Indeed, data from Antarctic ice cores* indicated that the previous three interglacials have lasted between 6000 and 9000 years which, if repeated, would have seen parts of Europe, Asia and North America covered in ice since before the rise of the Roman Empire. The most recent Antarctic ice cores have revealed that the warm phase before that lasted for 30,000 years. It is known, too, that the Earth’s alignment relative to the Sun during that long interglacial was similar to its alignment at the present time.

* An ice core is a sample obtained by drilling down into the ice cap. The state of the ice at different levels provides a climatic record that can extend over hundreds of thousands of years.

From the information in Doc 1, which of the following can reliably be concluded?

A Another ice age is due any millennium now.

B The standard view is wrong.

C The present warm phase is set to last another 20,000 years.

D According to the recent geological record, ice-age conditions are the norm, and it is Earth’s present climate which is unusual.

E Global warming is delaying the start of the next ice age.

Give a brief reason for each response.