In HOLLIS, use the 'Everything' search mode to find both books and articles. Use the filters on the right side of the screen to narrow your search results by date, language, subject, etc. For more control over your results, use the Advanced Search and limit your keywords to the title or subject.
Another good source for scholarly articles and book chapters is the database Philosopher's Index. If the database doesn't give you the full text for a particular article, use the blue "Try Harvard Library" button that appears with the citation.