“Whoever wants to be fair, let him imagine himself in his opponent’s place.”

From Ibn Hazm’s al-Akhlaq wa’l-Siyar, and the sentence finishes the thought: do that, and the unfairness of your own position will show itself to you. The whole book is made of tests like this one that he ran on himself and then wrote down, which is why it still reads as a practical manual more than nine centuries later.