The impression his mathematical training made on Jefferson was enormous. We see this both in the pattern of his career and particular achievements, and explicitly in the numerous letters he wrote throughout his life. Here, for example, is Jefferson in his early twenties, advising a younger man on the training he needed to become a lawyer [TJ to Bernard Moore c.1765]
". . . the faculties of the mind, like the members of the body, are strengthened and improved by exercise. Mathematical reasoning and deductions are, therefore, a fine preparation for investigating the abstruse speculations of the law."
Twenty years later we find him advising his future son-in-law on the role mathematics should play in his education [TJ to Thomas Mann Randolph, August 27, 1786]
The foundations which you have laid in languages and mathematics are proper for every superstructure. The former exercises our memory while that and no other faculty is yet matured, and prevents our acquiring habits of idleness, the latter gives exercise to our reason, as soon as that has acquired a certain degree of strength, and stores the mind with truths which are useful in other branches of science.
Twenty-five years after that, Jefferson was still paying testimony to his youthful enthusiasm, writing of his delight at helping his grandson with his mathematical studies and explaining just what it was about mathematics that he found so satisfying [TJ to Benjamin Rush, August 17, 1811]:
"Having to conduct my grandson through his course of mathematics, I have resumed that study with great avidity. It was ever my favorite one. We have no theories there, no uncertainties remain on the mind; all is demonstration and satisfaction."
The following year, in his seventieth year, Jefferson described his avidity in even more glowing terms [TJ to William Duane, October 1812]:
"When I was young, mathematics was the passion of my life."