The parent's point is that possession of a physical good is a bright line separation. For digital files, there's a huge difference between [Files you own] on a hard disk, and files [on a hard disk you own]. There are files you can put on a hard drive that you don't own and will ultimately kill themselves when specified criteria are met, like DRM'd ebooks.
replyI would argue that the files on your hdd that can expire or made unusable by some remote third party are as incomplete as a book that is missing half the pages. For example a keepass file without the password/key-file is incomplete, the same goes for Audible aax files that can not be played without per-user 'activation bytes.' You have possession of the file but you never owned its contents.
replyFor Audible I use OpenAudible which converts the aax files to m4b when I download them.
reply> The only difference between those files and, say, a DVD, is that the encoding is more space-efficient.
replyAlso that it's (depending on the format) perhaps not illegal to use the content in the file wit any viewer you choose.