Major meteor impacts and mass extinctions cannot be consistently causally linked unless many mass extinctions have
This is offering a rule about "consistent causal link", so it's worth considering. When we see "unless" we put a negated version of one of the two conditions on the left the arrow. Since we want the right side of the arrow to match our argument's conclusion, we'll negate the 2nd idea in this answer and throw it on the left side of the arrow. It is not the case that Major meteor impacts many mass extinctions ? and mass extinctions have followed major cannot be consistently meteor impacts causally linked The right side matches our conclusion. Does our evidence trigger the left side? Were we told that "almost no" mass extinctions ever follow major meteor impacts? No we were not. We were told that in many cases mass extinctions have not followed meteor impacts, but we weren't told there are "not many" cases where mass extinctions have followed impacts. Many doesn't have an official minimum, but we can think of it being something like "at least 10". When we negate many (and get not many), the best rephrase would be almost none. If I say "many of my friends have been to New York City" and you say, "That's not true", you're not committing to the idea that "NONE of my friends have been to NYC, just that I don't have many friends who have gone". So if you disagree with my statement, you're saying, "Not true. None (or almost none) of your friends have been to NYC." Thus, the trigger of this conditional (once we negate the quantity many) is saying, "If almost no mass extinctions have followed major meteor impacts, then ...". And we don't have any information to trigger that rule.