What analogy does the writer Samuel Johnson create in this excerpt from Rasselas?

I remembered that my father had obliged me to the improvement of my stock, not by a promise, which I ought not to violate, but by a penalty, which I was at liberty to incur; and therefore determined to gratify my predominant desire, and, by drinking at the fountain of knowledge, to quench the thirst of curiosity.

Question

What analogy does the writer Samuel Johnson create in this excerpt from Rasselas? 

I remembered that my father had obliged me to the improvement of my stock, not by a promise, which I ought not to violate, but by a penalty, which I was at liberty to incur; and therefore determined to gratify my predominant desire, and, by drinking at the fountain of knowledge, to quench the thirst of curiosity.

anonymous · Answer

Fountain of knowledge and thirst of curiosity.