What did Oliver Wendell Holmes say about obscenity?

The quote in question (paraphrased on the 9/29/10 Daily Show as "that which gives you wood"), refers to a 1964 supreme court decision by Justice Potter Stewart.

Although frequently attributed to Holmes, Potter Stewart wrote: "I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description ["hard-core pornography"]; and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that."