LSAT 155 - Section 1 - Question 23

This is a Weaken Except question, so that means four answers cut against assumptions that the argument made.

Usually when you see a Weaken Except question, that means the argument is especially bad because how else can there be so many assumptions for the answers to contradict? This argument is no exception. It really is bad. And it's bad for having made two different types of assumptions. This can be difficult to recognize. Another difficulty is the use of jargon and the reference to ratios. If it's one thing that LSAT students don't like, it's scientific jargon and math. Both are present here.

The first sentence states a causal relationship that occurs on Earth. On Earth, biological activity leads to, i.e., causes, a change in the ratio. So this is my advice about how to overcome the hurdle of jargon and also incidentally the hurdle of “math.” They don't matter. They don't matter because the rest of the argument and all the answer choices consistently reference the same “ratio.” So who cares what the ratio is called? All we need to focus on is the causal relationship, which is that biological activity causes a change in the ratio.

The next sentence tells us that a newly discovered meteorite, a rock, from Mars exhibits ratios found only in terrestrial minerals dating from before the beginning of life on Earth. The sentence takes a bit of parsing to understand. First, you have to understand that terrestrial minerals mean rocks on Earth. So, in other words, the ratio we find in this rock from Mars is similar to the ratio we find in rocks from Earth before there was life on Earth.

Now we get to the conclusion. The author concludes that it's unlikely life occurred on Mars.

I already said there are two different types of assumptions here. One is the assumption of analogy. This argument relies on reasoning by analogy because it assumes that the causal relationship on Earth of biological activity causing a change in the ratio would also be present on Mars. Would it? Mars and Earth are different places and those differences could mean that this causal relationship isn’t analogous. This is what Answer Choice (A) and Answer Choice (B) point out.

Answer Choice (B) says the effects of life on the ratio depend on a number of climatic and environmental factors with regard to which Earth and Mars differ. This is a very straightforward way of disanalogizing Earth and Mars. (B) tells us that biological activity isn't the only cause that's involved in the alteration of the ratio. Other causes matter too, like climatic and environmental factors, and those factors are different between Mars and Earth. So the ratio found in the Mars rock may not indicate the absence of life on Mars after all.

Answer Choice (A) is more subtle than (B) but also works on the argument’s reasoning by analogy. It says life forms that have a different effect on the ratio from that of life forms on Earth could have evolved elsewhere. This means that we shouldn't assume that Earth life forms’ effects on the ratio is universal. That means it's possible that different kinds of life forms could have evolved elsewhere and that those extraterrestrial life forms could have had a different effect on the ratio. This is an indirect way of suggesting that Mars and Earth are disanalogous. (A) is suggesting that if life had evolved on Mars, it's possible that Martian life would have had a different effect on the ratio.

Of the four answers that weaken the argument, these are the two that cut against the argument’s use of reasoning by analogy. They point out dissimilarities between Earth and Mars. (B) does this specifically and explicitly. (A) does this indirectly by suggesting that Earth and other places in general may be crucially dissimilar.

The other two answers that weaken the argument do so by cutting against a different assumption. That’s the assumption that the single Martian rock tells us something about the state of the Martian planet. If you think about it, it might occur to you that Mars is a big place and the meteorite is quite small by comparison. Is it true that the properties of that single rock reveal something about the entire planet? Well, that all depends on what properties of the rock we’re talking about and what characteristics of the planet we’re trying to figure out. In some ways, surely this rock is representative of Mars. But don't assume that it is representative of Mars in all ways. This is what Answer Choice (D) and Answer Choice (E) point out.

Answer Choice (E) says the current ratio on Mars is different from that at the time the meteorite left Mars. That means the ratio in the rock is not representative of the ratio on Mars today. That means this rock is not evidence of what has happened on Mars since it left the planet. Has life evolved in the intervening time? It’s unclear. (E) severely undermines the relevance of the only piece of evidence on which the conclusion is based by declaring the evidence to be chronologically unrepresentative.

Answer Choice (D) says that relatively few terrestrial mineral samples (rocks we find on Earth) contain ratios that would indicate the presence of life. This is a subtler way of calling out the representativeness of the rock from Mars. (D) says that if we looked at the ratios of rocks on Earth, we would find no signs of biological activity. Yet we know there is obviously plenty of biological activity on Earth. Therefore, this method of reasoning, that is, using the ratio found in rocks, is a poor way of figuring out whether there is life on Earth. This suggests that using this kind of reasoning might also lead to a faulty conclusion for Mars. I'm careful to say “suggests” because I recognize that this (meta) argument itself depends on an analogy between Mars and Earth. The crucial similarity assumed is that just like on Earth, even if there was biological activity on Mars, most of the rocks on Mars would not reflect that activity. That means there’s a good chance that this sample, this only piece of evidence we have, would also fail to capture the effects of life on Mars.

As you can see from the way these answers are structured, (B) and (E) are the more explicit refutations of the two assumptions in the argument. (A) and (D) are the subtler counterparts. They merely suggest that the assumptions are questionable.

Correct Answer Choice (C) says the ratio in the rock from Mars is the same as that on the planet as a whole at the time that the rock left Mars. This doesn't hurt the argument. This helps the argument, though only a little. Now we can be sure that this rock was representative of the ratio on Mars as a whole at some point in time. It doesn't guarantee that it's still representative of the ratio on the planet in the intervening time, as (E) points out, but it does at least partially patch up the issue of representativeness.