...infirmity alone makes us take notice and learn, and enables us to analyse mechanisms of which otherwise we should know nothing. A man who falls straight into bed night after night, and ceases to live until the moment when he wakes and rises, will surely never dream of making, I don't say great discoveries, but even minor observations about sleep. He scarcely knows that he is asleep. A little insomnia is not without its value in making us appreciate sleep, in throwing a ray of light upon that darkness. An unfailing memory is not a very powerful incentive to the study of the phenomena of memory.