 Sections  3.3  and  3.4  are more mathematically advanced. Section  3.3  introduces Cantor’s more mathematically sophisticated “cardinals and ordinals”, which are probably the mathematical developments that have done the most to untangle many of the conceptual confusions around infinity. This material is covered in greater detail in most textbooks on set theory, and parts of it are discussed in many logic textbooks as well. It can be read largely independently of the other sections.