Suppose you could send messages back in time to Archimedes of Syracuse, using a chronophone which - to avoid transmitting anachronistic information - transmits the results of executing cognitive strategies, rather than words. If you say "Women should have the vote", it comes out as "Install a tyrant of great personal virtue", because you repeated what your culture considers a wise form of political arrangement, and what comes out of the chronophone is the result of executing the same cognitive policy in Archimedes's era.
The chronophone won't transmit arguments you rationalize using your home culture's foreknowledge of the desired conclusion - it will substitute the result of executing that cognitive policy using Archimedes's culture's belief as the intended conclusion. A basic principle of the chronophone is that if you say something considered obvious in your home culture, it comes out as something considered obvious in Archimedes's culture.
The challenge was to say something useful under this restriction. This challenge is supposed to be difficult. It's really hard to get somewhere when you don't already know your destination. If there were some simple cognitive policy you could follow to spark moral and technological revolutions, without your home culture having advance knowledge of the destination, you could execute that cognitive policy today - which is what the whole parable is about!