Well, for most choices between "is this good?" and "is this bad?" the assumption is true. For example, is it good for me to drop my chocolate wrapper on the street instead of finding a rubbish bin? If I assume everyone were to do that, I get the idea of a street awash in chocolate wrappers, and I consider that reason enough to find a rubbish bin.
Furthermore, and more importantly, the aim here is not to produce an argument that one action is better than the other in a single, specific case; rather, it is to produce a general principle (whether it is generally better to be charitable to those nearby, or to those further away).
And if option A is generally better than option B, then I think it is very probable that universal application of A will remain better than universal application of B; and vice versa.
When you ask what it's like if everyone were to "do that", the answer you get is going to be determined by how you define "that". For instance, if everyone were to drop chocolate wrappers on the lawn of your annoying neighbor, you might be happy. So is it okay to drop the wrapper on your neighbor's lawn?
It's tempting to reply to this by saying "'doing the same thing' means removing all self-serving qualifiers, so the correct question is whether you would like it if people dropped wrappers wherever they wanted, not specifically on...
Another month has passed and here is a new rationality quotes thread. The usual rules are: