Thomasina is the girl genius of epic proportions. Thomasina intuitively knows the second law of thermodynamics and can refute determinism based on her ideas. Thomasina is a typical thirteen and then sixteen-year-old girl, except for the fact that she is unusually privileged and is given unusual educational opportunities. Although Lady Croom tells Thomasina that she must wed before she is overeducated, Lady Croom seems unconcerned at the intensity of her child's work until Thomasina nears the age of seventeen.

Thomasina is clearly driven not only by academic zeal but also by a desire for sexual knowledge. In the first scene, during her lesson with Septimus, Thomasina asks Septimus to tell her what a "carnal embrace" is. From the first pages of the book, Stoppard makes clear a duel purpose within Thomasina's character—to discover the rules of life and love while also working out the rules of mathematics. Thomasina's approach, including both carnal and academic knowledge, leads her to great success because she understands the principles of heat. Heat, which becomes equated with sexual knowledge, is the key to Thomasina's theory. Specifically articulated by Chloe, Thomasina's modern day counterpart, Thomasina's theory holds that sex messes up the Newtonian Universe because it is completely random.

Thomasina is ironically engulfed in the flame that she once seemed to understand better than anyone. Her tragic death, at the eve of her womanhood, drives Septimus to spend his lifetime tragically attempting to prove Thomasina's hypothesis. The final waltz that Thomasina and Septimus share at the end of play reveals a necessary urgency for sexual knowledge between all people. While the two talk about the end of the Earth, it seems Thomasina knows her end will be near. There is an understanding between tutor and student in the conclusion of the play; Thomasina and Septimus both understand the limits of and the ultimately unfulfilling nature of academic knowledge. Septimus and Thomasina dance and embrace to revel in the mystery they will never solve.