Logical reasoning PrepTest 147 · Section 1 · Question 4

Incorrect. The stimulus relies on the aliens contacting us, so the scientists focusing on a search for a particular kind of life isn't necessary for the argument to hold.

Incorrect. First, there being a reason to doubt these aliens exist is still compatible with both those aliens existing and them not existing. Second, this answer is about aliens more than 50ly from Earth, so it's out of scope.

Incorrect. The argument does rely on us being able to detect communication from these aliens—after all, if they're sending us messages and we're missing them, then the aliens might exist despite us not hearing their phone calls. However, the argument doesn't rely on scientists being able to "fully" understand the message.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Necessary Premise

Stimulus Summary:
Civ as advanced as humans within 50ly of Earth → Found evidence of us and easily contacted us
Therefore: not Civ as advanced as humans within 50ly of Earth

Answer Anticipation:
While it's an exceptionally long statement, that first sentence is conditional, establishing something that would be guaranteed to happen should there be a sufficiently advanced alien civilization relatively close to Earth. The argument then uses that conditional to conclude that the sufficient condition isn't true—that there isn't such a civilization.
An argument can validly conclude that a sufficient condition isn't true—the contrapositive tells us that missing the necessary condition guarantees that the sufficient condition is also missing:

not Found evidence of us and easily could have contacted us → not Civ as advanced as humans within 50ly of Earth

Here, however, the argument never establishes that the necessary condition isn't met, so the correct answer should address that shortcoming. Since this is a Strengthen with Necessary Premise question, the correct answer doesn't need to fully establish that condition is missing, instead only needing to bring up evidence that would be required for it to be true/missing.

Answer Explanation:
This answer is necessary for the lack of communication we've received from aliens to show that they don't exist. If these advanced aliens would want nothing to do with us, then a lack of communication doesn't guarantee that they don't exist.

Key Takeaway:
Two things.

First, when an argument relies on the application of a conditional to a specific case, it often fails to establish that the sufficient condition is met. In a Strengthen with Sufficient Premise question, that condition would need to be fully met, but in a Strengthen with Necessary Premise question, the correct answer just has to leave open the possibility that it can be met.

Second, note answer choice (B). It's common for the LSAT to throw a trap answer at you that has a subtle shift—often between phrases like "more than" and "less than" or related concepts like "rural" and "urban."

Incorrect. While the argument does require aliens to be able to detect signs of intelligent life on Earth, it doesn't require them to detect "all" such signs—just enough signs for them to know we're here.