I think the idea is "If everyone determined their actions as a function of the combination of their circumstances with their idiosyncratic wants, using the same function", where said function is assumed to be natural and not like, weirdly skewed around one particular person's wants.
So, like, if John really dislikes Joe, the rule saying "if you really dislike someone, punch Joe in the face" wouldn't be a rule of the appropriate sense. It doesn't make sense that if Steve really disliked Erwin, that he would punch Joe in the face as a result.
The rule that John should be considering whether he would approve of, is closer to[1] the rule "If you really dislike someone, punch them in the face." . Presumably John would find this rule less appealing than "If you really dislike someone, punch Joe in the face.", because someone might dislike John and therefore punch John in the face, and John would rather not be punched in the face.
So, in the case you gave an example of, about "whether to have children", the question would be along the lines of, "If everyone decided whether to have children (as well as how many) using the same criteria that you are using (where these criteria could include their personal tastes, aspirations, etc. as a factor), would you approve of the outcome?"
This seems quite compatible with the ways that people differ.
Like, it isn't about assuming everyone behaves in the same way, but about assuming everyone responds to their desires and motivations in "the same way", for the right sense of "the same way".
[1] I won't say that this is exactly the right rule to consider. idk. Probably not quite.
So, like, if John really dislikes Joe, the rule saying "if you really dislike someone, punch Joe in the face" wouldn't be a rule of the appropriate sense. It doesn't make sense that if Steve really disliked Erwin, that he would punch Joe in the face as a result.
The rule that John should be considering whether he would approve of, is closer to[1] the rule "If you really dislike someone, punch them in the face." . Presumably John would find this rule less appealing than "If you really dislike someone, punch Joe in the face.", because someone might dislike John and therefore punch John in the face, and John would rather not be punched in the face.
So, in the case you gave an example of, about "whether to have children", the question would be along the lines of, "If everyone decided whether to have children (as well as how many) using the same criteria that you are using (where these criteria could include their personal tastes, aspirations, etc. as a factor), would you approve of the outcome?" This seems quite compatible with the ways that people differ.
Like, it isn't about assuming everyone behaves in the same way, but about assuming everyone responds to their desires and motivations in "the same way", for the right sense of "the same way".
[1] I won't say that this is exactly the right rule to consider. idk. Probably not quite.