What is number theory?  The image displays a YouTube screen showing a video about "What is number theory?" and its comment section. The video's description defines number theory as: "Number theory is the branch of mathematics that's primarily concerned with our counting numbers, 1, 2, 3, .... Particularly important are prime numbers, the 'building blocks' of our number system, whose only factors are themselves and 1: prime: 11, 13, 17, 19 non-prime: 15 = 3 x 5, 18 = 2 x 9 = 2 x 3 x 3, 91 = 7 x 13, 323 = 17 x 19 2047? 30,031? 4,294,967,297?" Below the video description, there's a comment section with "Comments 251" visible. One comment, posted by "@AlQ8" "0 seconds ago," reads: "Perhaps too 'brief.' I'd have started with arbitrary human decisions like pentadactyly and base-10, and subsequent obsessions with finding order in such arbitrariness through such imagined distinctions as odd/even and prime numbers... But what do they mean?? Whatever we construct our definitions to mean! That's number theory to me."