LSAT 144, Section 1, RC Passage 4, Negative Evidence

This is an explanation for passage 4 of LSAT preptest 76, the October 2015 LSAT – the comparative passage. The passages are about the role of negative evidence in scientific research. The first passage discusses how negative evidence is not as conclusive as Popper believed. The second passage gives us two examples of how negative evidence was used in practice.

This section has paragraph summaries and an analysis of the passage, links to the explanations for the questions are below.

Paragraph Summaries

Passage A

1. Popper says that positive evidence can never be conclusive, but that a single piece of negative evidence disproves.
2. However, all theories depends on auxiliary premises. When a prediction fails, it’s not clear if the theory was wrong or if an auxiliary premise was wrong.

Passage B

1. Uranus’ orbit didn’t follow predictions. This could have meant Newton’s theory was wrong, or an auxiliary assumption was wrong. It turns out Neptune existed, disproving the auxiliary assumption that there was no other planet. Newton’s theory was therefore still considered correct.
2. Scientists tried to repeat the process with Mercury, but the planet Vulcan was never found. Scientists rejected Newton’s theory itself after Einstein’s theory + the auxiliary assumptions predicted Mercury’s orbit.

Analysis

Both passages are broadly in agreement, with some minor differences. The first passage is more theoretical, and the second passage gives examples.

Both passages talk about “negative evidence”. Passage A mentions it directly, and passage B uses it indirectly through examples. Negative evidence is evidence inconsistent with a theory. So if I say “The sun shines every day, always”, then there is a lot of positive evidence for my theory, because the sun usually shines. But one day, there will be a solar eclipse and the sun won’t shine – that’s negative evidence which goes against my theory.

I’ll be using negative evidence a lot, so make sure you understand what it means. It’s evidence inconsistent with a theory. Now, on to the passages.

Passage A Analysis

Popper was a philosopher who said that science could never confirm, only disconfirm. For example, once upon a time in Europe, people had only ever seen white swans. Was it valid to say that all swans were white because all swans people had seen were white?

Popper says no. Because it would take only a single counterexample to disprove the claim. And in fact, once Europeans reached Australia, they found black swans, which disproved the claim that “all swans were white”.

(This general problem is called “the problem of induction” if you’d like to read more. It’s central to the LSAT.)

Popper took this idea and pushed it to its limits in science. It’s now generally accepted that no theory can be proven true by evidence. We can only say “We’re fairly confident in this theory, because it hasn’t been disproven yet”.

The author of passage A largely agrees with Popper. However, their criticism is that negative evidence isn’t quite so powerful as Popper says. Popper claimed that if there was a negative result, then a theory was wrong.

But in practice, usually you have a theory + several other assumptions. If a result isn’t as expected, the theory could be wrong, but it could also be that one of the auxiliary assumptions is wrong.

For instance, let’s say my theory is that I have a magic rock that guarantees a 180 on the LSAT. But in practice, you still need to do some stuff:

• Register for the LSAT
• Actually go take it
• The results have to be processed by LSAC
• You have to survive until taking the LSAT, etc.

So if I give you a rock, but then you don’t get a 180, my theory about the magic rock might be wrong. But it’s also possible an auxiliary assumption could be wrong. For instance, you might have won the lottery, and decided you didn’t need to work, so you didn’t take the LSAT at all. But the rock could have worked, had you taken the LSAT.

So I can’t say the rock didn’t work just because you didn’t get a 180. I’d have to know whether all the auxiliary assumptions were true too. Popper is right that one of either the theory or the auxiliary assumptions is false, but we can’t say which one.

Passage B – Practical Examples

Passage B has a different emphasis. It gives us two examples of how negative evidence was used in practice.

First, some background. Newton’s theory was an extraordinarily accurate theory of physical laws. It’s so accurate that we still use it today for many calculations where its inaccuracies don’t matter.

Einstein’s theory was better than Newton’s in a few areas, so Einstein’s theory is now accepted as the better theory.

The two examples had different results. In the case of Uranus, Newton’s theory wasn’t rejected; only an auxiliary assumption was. In the case of Mercury, Newton’s theory itself was rejected. This illustrates the argument in passage A that negative evidence doesn’t necessarily invalidate a theory.

Uranus and Neptune: Scientists tried to predict Uranus’ orbit. They used Newton’s theory to do this, along with several other assumptions, including “there is no other planet near Uranus”.

But their predictions failed. Rather than discard Newton’s theory, the scientists assumed it was more likely that their assumption was wrong. So they searched for another planet and found Neptune.

So in this case, negative evidence discarded an auxiliary assumption, rather than the main theory.

Mercury: The same situation occurred with Mercury. Scientists originally used Newton’s theory and also the auxiliary assumption “there is no other planet near Mercury that we don’t know about”.

But when the Newtonian predictions of Mercury’s orbit failed, scientists initially discarded this auxiliary assumption and searched for another planet affecting Mercury’s orbit. (They named this predicted planet “Vulcan”)

However, there was no planet Vulcan, and the orbit predictions were still wrong. Scientists concluded that Newton’s theory itself might be wrong.

Then later Einstein introduced his laws of relativity. These correctly predicted the orbit of Mercury. So scientists discarded Newton’s theory and had more confidence in Einstein’s theory.

So this is an example of discarding a theory itself, rather than an auxiliary assumption. Scientists kept the auxiliary assumption that “there is no planet near Mercury”.

Note the final line of the passage: “and to increased confidence in Einstein’s theory”. This is a difference from passage A. Popper thought positive evidence could never confirm. Whereas the author of passage B says that scientists did view the correct prediction of Mercury’s orbit as some sort of proof for Einstein’s theory.

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