I think most of the confusion about white hat and black hat SEO (I prefer to think of it as a spectrum of grey rather than in absolutes) revolves around links, or more specifically link schemes. Just read this excerpt from the Google Webmaster Guidelines:
Any links intended to manipulate PageRank or a site's ranking in Google search results may be considered part of a link scheme and a violation of Google?s Webmaster Guidelines. This includes any behavior that manipulates links to your site or outgoing links from your site.
After reading that paragraph, you're bound to wonder exactly what a link scheme is, especially if you know the definition of scheme.
Is doing on-page SEO to get links considered link scheming?
Is sharing your link on social media in hopes of getting do-follow links considered link scheming?
Is writing great content in hopes of getting a do-follow link considered link scheming?
Ranking a site with "white hat only" tactics is possible, it just takes much longer since the "white hat" way of "building backlinks" would be to use the method in my Website ranking guide, which is to rank a website using on-page SEO first and procure backlinks, although again, this might be "link scheming" (albeit a more difficult-to-prove link scheme).