Source: Logan R. Ury. Burden of Proof. Dec 6, 2006. Wikipedia on Math 55.
While Harrison ultimately chooses to remain in the class, such conferences motivate several more students to drop, including the only two females: [Elizabeth A] Cook ['10], who had looked forward to taking the class, and Laura P. Starkston ’10. “The problem was that I wasn’t prepared to think that abstractly,” Cook says. “Gaitsgory pointed it out to me in our private conference. Eventually I just got the picture.”
What exactly does 'abstract' signify here? What makes the thinking 'that abstract'?