"Being the third son of the family, and not bred to any trade"

During the 17th and 18th centuries, sons of established families were usually groomed for particular careers depending on their order of birth and family circumstances. The first son was heir to the family estate or business, but if the family was not wealthy he would often pursue a military career. The second son invariably went into the church.  The third (like Crusoe) would usually having some respectable trade chosen for him.