You are a space alien, minding your business, cruising the galaxy when you encounter an anomolous object. Upon inspecting the object you notice that it contains a gold phonograph disc with symbols etched upon the cover, which is electroplated with pure Uranium 238.
The disc was designed 100,000 years earlier by an Earth astronomer who anticipated this momentous occasion, and sent the disc into space aboard a spacecraft. The astronomer was counting on two things. First, that an advanced spacefaring civilization would find it; and second, that they would have surpassed a developmental milestone which would allow their civilization to determine that the object and the disc were purposefully designed.
What developmental milestone did the astronomer hope that your alien civilization had achieved?
A) Invention of the phonograph.
B) Development of symbolic language.
C) Invention of radiometric dating.
D) Development of critical thinking.
E) Invention of science.
Answer below the fold.
If you answered D) Development of critical thinking you are correct.
While the phonograph would have been a fortuitous coincidence, it would not be necessary for your people to detect design.
Symbolic language would be helpful in decoding the message on the disc, but this is separate from the detection of design.
Radiometric dating would be a useful archaeological tool, but not crucial to design detection.
Science, burdened by the requirement for objective independent knowledge of the designer would be hopelessly impotent in detecting design.
Critical thinking is the only known tool that can detect design in such an instance. Any civilization that was incapable of critical thinking would have no basis for inferring that the disc was designed with a purpose.