Once again a case of your feet in my Nike's
Before you criticize somebody, try walking a mile in (his|her) shoes
Lots of variants on the quote, but the point is that it’s easy to defame or mischaracterize somebody through criticism when, in fact, one knows nothing about the situation, circumstances, history and motivations of the target of the criticism.
In this case, Phife is saying, “you’re not in my Nike’s, so your criticism is invalid”.
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