PrepTest 102

Incorrect. The argument establishes that we can't measure both of these things because, without both of them, we can't determine the current state of the universe, or where particles are going to be next. As such, the Physicist doesn't need it to be the case that we can't figure out just the position or just the velocity of given particles.

Incorrect. The argument uses the inability to measure particles to support the claim that we can't know the complete state of the universe, so this answer reverses the logic of the stimulus.

Incorrect. The argument doesn't claim that they can't be measured separately. But if they are, then they're being measured at different times, and so you wouldn't know the other at that time, still missing some information about the complete state of the universe.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Errors in Reasoning

Stimulus Summary:
Determinism - Any event was the necessary outcome of what happened right before

But it's impossible to know the position and velocity of particles, so you can't know where they all are, so determinism is false.

Answer Anticipation:
This argument falls into the rebuttal structure—a theory is put forward, and the Physicist concludes that it's false. We should analyze the rebuttal to identify an error in it.

And the argument itself uses very strong language, and gets pretty philosophical on us, but there are two concepts we can home in on that suggest specific flaws.

First, the Physicist discusses two things that are "impossible"—measuring two things simultaneously, and knowing the complete state of the universe. However, in attempting to disprove something, strong language isn't a problem—in fact, the stronger the language, the more likely it is to be able to disprove a theory. So this isn't related to the flaw.

Second, the Physicist discusses measuring and knowing parts of the universe. However, the conclusion doesn't care about subjective knowledge—it cares about objective truth. Is it the case that the state of the universe inevitably leads to the next events that occur? That's what determinism says, and that can be true even if we can't possibly measure everything and know what the next events are going to be.

In shifting between a premise about what is impossible to "know" to a conclusion about a theory of what is being false, the argument is making a Perception vs. Reality flaw. Let's find an answer pointing that out.

Answer Explanation:
This answer highlights the Perception vs. Reality flaw. The Physicist establishes that the complete state of the universe can't be measured/known, but that doesn't mean that it can't exist. Since Determinism requires only that there is a complete state of the universe, not that the state can be known, the rebuttal fails. This answer is therefore correct.

Key Takeaway:
Knowing language/concepts related to flaws on the LSAT can be really important and anticipating answers to questions like this one. This argument is technical and philosophical, and noting that it brings up what we can "know" is a great way to quickly end up spotting the Perception vs. Reality flaw present.

Incorrect. That's exactly what it means—if it can't be done for any given particle, then no individual particle can have both of these things measured.