Summary
The author concludes that there is no consistent causal link between major meteor impacts and mass extinctions. This is the based on the following:
Many extinctions didn’t follow major meteor impacts.
Many major meteor impacts were not followed by mass extinctions.
Many extinctions didn’t follow major meteor impacts.
Many major meteor impacts were not followed by mass extinctions.
Missing Connection
The premises don’t establish what implies that there’s “no consistent causal link” between major meteor impacts and mass extinctions. Although we know that many extinctions didn’t follow such impacts, and that many impacts weren’t followed by such extinctions, how do we know that this isn’t a consistent causal link?
We want to establish that if many extinctions didn’t follow major meteor impacts or if many major meteor impacts were not followed by mass extinctions, then this proves there’s no consistent causal link between such impacts and such extinctions.
We want to establish that if many extinctions didn’t follow major meteor impacts or if many major meteor impacts were not followed by mass extinctions, then this proves there’s no consistent causal link between such impacts and such extinctions.
A
If there were a consistent causal link between major meteor impacts and mass extinctions, then all major meteor impacts would be followed by mass extinctions.
(A) establishes that in order for there to be a consistent causal link between the impacts and mass extinctions, then ALL major meteor impacts would be followed by such extinctions. We know from one of our premises that many major meteor impacts were NOT followed by such extinctions. This triggers the contrapositive of (A), which would establish that there is NO consistent causal link.
B
Major meteor impacts and mass extinctions cannot be consistently causally linked unless many mass extinctions have followed major meteor impacts.
(B) establishes that in order for there to be a consistent causal link, it’s required that many mass extinctions have followed major meteor impacts. But we don’t know that this requirement hasn’t been met. Although we do know that many mass extinctions have followed major meteor impacts, it’s still possible that many such extinctions have followed such impacts.
C
Of the mass extinctions that did not follow any known major meteor impacts, few if any followed major meteor impacts of which the geological record contains no hints.
(C) doesn’t establish what is required for a “consistent causal link” between extinctions and meteor impacts, so it can’t make the argument valid.
D
If there is no consistent causal link between major meteor impacts and mass extinctions, then not all mass extinctions could have followed major meteor impacts.
(D) establishes what would be true IF there is no consistent causal link. But we’re trying to reach the conclusion that there is no consistent causal link. We don’t want “no consistent causal link” to appear in the “IF” part of an “If, then” answer. We want it to appear in the “then” part.
E
There could be a consistent causal link between major meteor impacts and mass extinctions even if not every major meteor impact has been followed by a mass extinction.
(E) tells us about one circumstance under which there can be a consistent causal link. But we’re trying to prove that there’s NO consistent causal link.
Summarize Argument: Phenomenon-Hypothesis
The author concludes that dioxin released from paper mills is unlikely to be the cause of reproductive abnormalities in fish immediately downstream of the mills. This is because when the mills shut down, the fish recover normal hormone concentrations relatively quickly, even though dioxin decomposes very slowly in the environment (which suggests the dioxin didn’t just disappear quickly from the area).
Notable Assumptions
The author assumes that the dioxin is still present around the fish immediately downstream of the paper mill during the mill shutdowns. This is why the author thinks the quick recovery of the fish shows that dioxin isn’t likely to be the cause of the fish’s problems. The author also assumes that the reproductive abnormalities in the fish are caused by abnormal hormone concentrations.
A
Some of the studies that show that fish recover quickly during shutdowns were funded by paper manufacturers.
Whoever funded the studies doesn’t change anything about what the studies found. We have no reason to think the source of funding affected how the study was done in a way that would lead us to question the results.
B
The rate at which dioxin decomposes varies depending on the conditions to which it is exposed.
A premise already establishes that dioxin decomposes very slowly in the environment. Although the rate might not be exactly the same in all environments, we still know it decomposes very slowly. So, (B) doesn’t suggest that the dioxin might have decomposed quickly.
C
Normal river currents carry the dioxin present in the river far downstream in a few hours.
This shows dioxin might not be around the fish anymore during a shutdown. This is why the recovery of the fish during a shutdown does not constitute strong evidence that dioxin isn’t the cause. If dioxin isn’t around the fish at these times, that might be the reason fish recover.
D
Some of the fish did not recover rapidly from the physiological changes that were induced by the changes in hormone concentrations.
One assumption is that the reproductive abnormalities are caused by hormone concentrations. Even if we interpret “physiological changes” as including reproductive abnormalities (which is not clear), this affirms a link between hormones and reproductive abnormalities.
E
The connection between hormone concentrations and reproductive abnormalities is not thoroughly understood.
The fact the connection is not thoroughly understood does not undermine the assumption that hormone concentrations cause reproductive abnormalities. The issue is not how well we understand the relationship; it’s about whether there exists a causal relationship.