I've always had a hard time shopping for Dan. Not only is he an introverted internal processor, he's also very hard to read. Sometimes I think I've got it just right only to find out it's the wrong shape, color, or he just flat out doesn't want it.
Now I know, it's money he's really after.
Let me explain. Dan came across this hilarious and very accurate blog post about the Do's and Don'ts of buying philosophers presents. The first two presents a philosopher wants is money and gift cards which they will then spend on food and books. (Notice that books are NOT something you should get a philosopher because an ordinary person will never be able to find one obscure enough.) Money also helps such philosophers to eat, seeing as how they rarely get jobs being philosophers.
I especially like number 3 in the "DO" category. The pen thing was right on.
Ok, philosophers love pens, for some reason. They all have their particular favorites, though, so ask then first about which type they like. You could buy them pens at every special occasion and they will be extremely satisfied.