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191 pages, Kindle Edition
First published June 21, 2016
There's no problem with appealing to authority when that alleged authority is in fact an expert on the topic in question. So, if I say, "Black holes emit radiation," I can justify this by appealing to the authority of Stephen Hawking.
First, point out that your opponent's argument is simply invalid: the fact that one difficult thing has been achieved doesn't mean that a different difficult thing may also be achieved. After all, the difficulties associated with the latter feat remain unaffected by the achievement of the first feat. You should then point out just how great these difficulties are; perhaps putting a man on the moon is in fact relatively simple compared to curing cancer.