— Tyrion Lannister (Ch 5, A Game of Thrones (Book 1 of 7 in the A Song of Ice and Fire series))
I, personally, find a lot of truth in this analogy. I would also add that a poor-quality whetstone will not do its job so well as a good one; this is why editors and proofreaders have their jobs, and why these jobs are so important. A book with poor spelling, grammar and punctuation will not help to sharpen anyone’s mind, as they will have to expend more mental energy on trying to work out what the text is trying to say than on engaging with the ideas contained therein. Editors and proofreaders make the text as good as it can be (without interfering with the voice of the author) so that the reader can more easily interpret it and maintain their mind’s keen edge.
At risk of overextending the book/whetstone analogy, I will stop here.