BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

Target Test Prep · GMAT

Choose how you want to prepare

Learn live with an expert or move at your own pace. Every option includes the complete TTP study system.

★★★★★5.0559 reviews
Vote for Target Test Prep, Newsweek Readers’ Choice Awards 2026
NEWSWEEK READERS’ CHOICE 2026

BIG NEWS! Target Test Prep has been nominated, and they’d love your vote!

TTP has worked incredibly hard to build the best test prep experience possible, and winning Newsweek’s 2026 Readers’ Choice Award for Best Test Prep would mean a lot to them. If TTP has helped you, they’d be incredibly grateful for your vote. You can vote once each day through September 9.

Vote for TTP
GMATLiveTeach 7 seats left
Chris Peckover
NEXT LIVE COHORT

Oct 13 to Jan 7, 2027

with Chris Peckover

Schedule
Tue, Thu · 8:00 to 10:00 PM ET
Included
40 live hours + 6 months of GMAT OnDemand
  • Live instruction and real-time questions
  • Class recordings and assigned practice
View class & enroll
Limited cohort · enrollment openTarget Test Prep
EALiveTeach 5 seats left
Logan Thompson
EXECUTIVE ASSESSMENT

Sep 6 to Dec 6, 2026

with Logan Thompson

Schedule
Sun · 9:30 AM to 12:30 PM ET
Included
40 hours of live online classes plus six months of access to the complete TTP EA OnDemand course.
  • 165+ EA Score Guarantee
  • 4,100+ Quant, Verbal, and Integrated Reasoning practice questions
  • 400+ hours of in-depth video lessons
  • 3,000+ step-by-step video solutions
View EA class & enroll
Limited cohort · enrollment openTarget Test Prep
GMATOnDemand Start anytime
SELF-PACED MASTERCLASS

Target Test Prep GMAT OnDemand

Complete access from day one. Study on your schedule.

715+ score guarantee
$0to start then $127/mo
  • Personalized study plan and analytics
  • Thousands of lessons and practice questions

Compare the format, schedule, and included access before enrolling. Prices and seat counts shown reflect the supplied offer details.

theory is either true

Expert replies
by sogmat » Thu Dec 18, 2008 9:35 am
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. Therefore, since the Copernican theory of planetary motion is inconsistent with the Ptolemaic account, Galileo’s observations of Jupiter’s satellites proved the truth of the Copernican theory.
The argument above is open to the objection that it makes the questionable assumption that
A. whoever first observed something inconsistent with the truth of the Ptolemaic theory should be credited with having proved that theory false
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
C. the Ptolemaic and Copernican theories, being inconsistent, cannot both be based on exactly the same evidence
D. numerous counterexamples were necessary in order to show the Ptolemaic theory To be false
E. the Ptolemaic and Copernican theories, being inconsistent, cannot both be false

OA ([E][/spoiler]
Join the discussion
Source: — Critical Reasoning |

by BTGmoderatorLU » Sat Oct 21, 2017 3:03 pm
I don't understand it. Can any explain this please.
Join the discussion

by EconomistGMATTutor » Mon Oct 30, 2017 10:41 am
The conclusion: Galileo's observations of Jupiter's satellites proved the truth of the Copernican theory.

The evidence: 1) A theory is either true or false.
2) Galileo's observations of Jupiter's satellites showed that the Ptolemaic theory of the motion of celestial bodies is false.
3) the Copernican theory of planetary motion is inconsistent with the Ptolemaic account.

We know that Ptolemy was wrong. We know that Copernicus is inconsistent with Ptolemy. That doesn't mean that Copernicus was correct.
Let's say Dane thinks that 2 + 2 = 5. We know that Sal's opinion is inconsistent with Dane's on this subject. That doesn't mean that Sal is correct. He might think 2 + 2 = 13. Dane and Sal are inconsistent with each other, but they're both wrong. Of course, Sal would be correct if he did know that 2 + 2 =4, but we don't know that. We also don't know what Copernicus thinks, and we can't assume what he thinks. Ptolemy and Copernicus could be inconsistent with each other (which would have been a better way of wording Choice E) and still both be wrong.

Choice A brings in a number of irrelevant pieces of information.
Choice B doesn't add anything new.
Choice C does not need to be assumed. They could have reached different conclusions based on the same evidence.
Choice D is irrelevant background information.

I'm available if you'd like any follow up.
GMAT Prep From The Economist
We offer 70+ point score improvement money back guarantee.
Our average student improves 98 points.

Image
Join the discussion