I never read Lovecraft as being any kind of metaphor for the real world, so I wouldn't vote this up as a rationalist quote for that reason.
But I like it as a device used Lovecraft to try to convey a sheer magnitude of horror. Can you imagine discovering something so horrific you wished you could delete the whole thing from your memory ? The more you pride yourself as a rationalist, the more horrific it would have to be.
This is our monthly thread for collecting these little gems and pearls of wisdom, rationality-related quotes you've seen recently, or had stored in your quotesfile for ages, and which might be handy to link to in one of our discussions.