Sentence:  It is not until section 7 that we deal with <WSD>commutative algebra</WSD> proper, whereas the sections leading up to it should be seen as advocacy that excursions into noncommutative algebra can help to shed light on problems in <WSD>commutative algebra</WSD>. 
Meaning of the word marked with <WSD> tags:  The branch of algebra concerned with commutative rings and objects related to them (such as ideals and modules).