Hi everyone, I m a college student taking Philosophy and I can t understand the gist of this excerpt, an explanation would be wonderful :)?

"If you point out to me that my act will seriously harm your interests, you give me good reason to believe that my act is wrong. If the fact that my act will seriously harm your interests is a good reason for me to believe that it is wrong, then the fact that your act will harm my interests is a good reason... show more "If you point out to me that my act will seriously harm your interests, you give me good reason to believe that my act is wrong. If the fact that my act will seriously harm your interests is a good reason for me to believe that it is wrong, then the fact that your act will harm my interests is a good reason for you to believe that your act is wrong. Yet, if your act s effects on my interests are relevant to whether it is right or wrong, then Ethical Egoism is mistaken."

I understand the first two sentences completely, but the third one is sort of tricky and I m having trouble wrapping my head around it. As always, I really appreciate any help.
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