A theory is either true or false. Galileo's observations of

Lucy Phuong,
Yes, we are looking for a questionable assumption that the argument makes. The author states that "the Ptolemaic theory of the motion of celestial bodies is false" and that "the Copernican theory of planetary motion is inconsistent with the Ptolemaic account". Since "a theory is either true or false", the author concludes that the Copernican theory must be true. According to the author, since the Ptolemaic theory is false and the Copernican theory is inconsistent with the false Ptolemaic theory, the Copernican theory must be true. But this reasoning fails to consider the possibility that the Ptolemaic theory and the Copernican theory are BOTH false, a scenario that satisfies the given condition that a theory is either true or false. In other words, just because the theories are different, does not mean that one is false and one is true; it is possible for both theories to be false, even if those theories are different from one another.

is this really a 700 level question? just want to understand the range of difficulty that i should expect on the official test.

compared to many questions i have seen in the directory this seemed way easier than the other 700 level questions, thats why im curious.

thank u!

You can check the difficulty level of a question in the tags just above the first post. You can also check the stats in the original post. For this question the difficulty level is 700+.

The background states that since Galileo’s observations proved that the Ptolemaic theory (Theory P) is false, the Copernican theory (Theory C) MUST be true since the two theories are inconsistent with each other. Nowhere in the information has it been stated that ‘Theory P and Theory C are the ONLY theories accounting for Galileo’s observations'. In essence, an assumption can be thought of as a strengthener of the argument, since without it holding true the entire argument would fall apart.

A. Whoever first observed something inconsistent with the truth of the Ptolemaic theory should be credited with having proved that theory false.

The argument is focused on the truth of either theories and not on the individual credited with proving/disproving a theory, so this answer choice is irrelevant.

B. There are some possible observations that would be inconsistent with the account given by the Copernican theory but consistent with the account given by the Ptolemaic theory.

This choice could be tempting but the background information clearly talks about the observations of Jupiter by Galileo and not any other observations. Importantly, this seems to weaken the conclusion that the argument holds to be true.

C. The Ptolemaic and Copernican theories, being inconsistent, cannot both be based on exactly the same evidence.

Again, if we consider this choice to be true, the argument falls apart since the inconsistency of the theories and their consequences are both based on the Galileo’s observations (same evidence) in this case. Anyway, we are concerned with the truth of the theories and not their underlying reasons so this answer choice can be eliminated.

D. Numerous counterexamples were necessary in order to show the Ptolemaic theory to be false.

Even if numerous counterexamples were required to show Theory P to be false, this fact in no way affects the truthfulness of Theory C. Applying the negation test to this answer choice does not have any effect on the conclusion of the argument, hence this choice is incorrect.

E. The Ptolemaic and Copernican theories, being inconsistent, cannot both be false.

Finally something in line with our analysis! There is no justification in the background information that either of the two theories must be correct so if the conclusion is to stand, this answer choice must be assumed and negating this answer choice destroys the conclusion. Therefore, choice (E) is our answer.

Understanding the argument -
A theory is either true or false.
Galeleo's observations of Jupiter's satellites showed that the Ptolemaic theory of motion is false.
Ptolemaic theory is inconsistent with Copernican theory.
So, Galeleo's observations of Jupiter's satellites showed the truth of Copernican theory.

Though crystal clear, the argument can be bewildering or confusing if read in a hurry. So read carefully and connect the sentences. The argument is based on a key assumption: one of the theories has to be true, or both can't be false simultaneously. Because if both are false at the same time, then in this case, we can't say with 100% confidence that falsifying Ptolemaic's theory automatically means that Copernican theory is true.

There could also be another assumption, which is that there can be, say, 20 other theories than Copernican theory and Ptolemaic theory, but as the argument is limited to these two theories, we can ignore this assumption for now.

Option Elimination -

(A) Whoever first observed something inconsistent with the truth of the Ptolemaic theory should be credited with having proved that theory false. - Whosoever gets credit is not even an issue in the argument. Out of scope.

(B) There are some possible observations that would be inconsistent with the account given by the Copernican theory but consistent with the account given by the Ptolemaic theory. Then what? Just use similar words from the argument and arrange them to create some random option. Distortion.

(C) The Ptolemaic and Copernican theories, being inconsistent, cannot both be based on exactly the same evidence. - At best, it is a weakener. It's trying to say that both The Ptolemaic and Copernican theories can't be based on the same evidence. But here, we use the same evidence to establish the truth of Copernican theory and falsify Ptolemaic theory.

(D) Numerous counterexamples were necessary in order to show the Ptolemaic theory to be false. - No. We just used one example as shared in the argument. What happened other than the scope of the argument given is none of our business. Out of scope.

(E) The Ptolemaic and Copernican theories, being inconsistent, cannot both be false. - ok. If we negate this option, it shatters the conclusion.

Conclusion: Galileo’s observations of Jupiter’s satellites proved the truth of the Copernican theory.
Premise: A theory is either true or false. Galileo’s observations of Jupiter’s satellites showed that the Ptolemaic theory of the motion of celestial bodies is false.
Assumption: Since Ptolemic theory is false, it cannot happen that the copernican theory which is inconsistent with this theory is also false.
If Galelio's theory proves a theory false, then it must have proved the correctness of a theory which is inconsistent with the first theory.

Why correct?(E) The Ptolemaic and Copernican theories, being inconsistent, cannot both be false. --> Similar to assumption. If both theories are false, it undermines the conclusion that Galelio's actually proved the truth copernican theory by proving the ptolemic theory as false.
Why wrong? (A) Whoever first observed something inconsistent with the truth of the Ptolemaic theory should be credited with having proved that theory false. --> The argument is not about which theory (Galelio's or copernican) first contradicted Ptolemic theory. Hence, out of scope

(B) There are some possible observations that would be inconsistent with the account given by the Copernican theory but consistent with the account given by the Ptolemaic theory. --> This is opposite to the assumption we have. This will weaken the conclusion hence cannot be the assumption.

(C) The Ptolemaic and Copernican theories, being inconsistent, cannot both be based on exactly the same evidence. --> No such claim is being made in the argument. Even with the same evidence, interpretation of theory can be different, which doesn't have an impact on the argument.

(D) Numerous counterexamples were necessary in order to show the Ptolemaic theory to be false. --> Irrelevant as doesnt answer how galelio proved the copernican theory right.