I think it's most worth noting that "I forgot Obama was black"--in all its iterations--is something that white people should stop saying, if only because it's really dishonest. One way to think about this is to flip the frame. Around these parts, we've been known, from time to time, to chat about the NFL. We've also been known to chat about the intricacies of beer. If you hang around you'll notice that there are no shortage of women in these discussions. Having read a particularly smart take on Brett Favre, or having received a good recommendations on a particular IPA, it would not be a compliment for me to say, "Wow, I forgot you were a woman." Indeed, it would be pretty offensive.(Emphasis mine. The wisdom, however, is all his.)
The problems is three-fold. First, it takes my necessarily limited, and necessarily blinkered, experience with the fairer sex and builds it into a shibboleth of invented truth. Then it takes that invented truth as a fair standard by which I can measure one's "woman-ness." So if football and beer don't fit into my standard, I stop seeing the person as a woman. Finally instead of admitting that my invented truth is the problem, I put the onus on the woman. Hence the claim "I forgot you were a woman," as opposed to "I just realized my invented truth was wrong."
Ditto for Chris Matthews. The "I forgot Obama was black" sentiment allows the speaker the comfort of accepting, even lauding, a black person without interrogating their invented truth. It allows the speaker a luxurious ignorance--you get to name people (this is what black is) even when you don't know people. In fact, Chris Matthews didn't forget Barack Obama was black. Chris Matthews forgot that Chris Matthews was white.
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