Jewel collectors, fearing that their eyes will be deceived by a counterfeit, will not buy a diamond unless the dealer guarantees that it is genuine. But why should a counterfeit give any less aesthetic pleasure when the naked eye cannot distinguish it from a real diamond? Both jewels should be deemed of equal value.
The principle 'It should not be assumed that everyone who likes diamonds receives the same degree of aesthetic pleasure from them' if valid, most helps to justify the reasoning in the argument above.