| Quote #1
Thomasina: There is no proof, Septimus. The thing that is perfectly obvious is that the note in the margin was a joke to make you all mad. (1.1)
The "joke" Thomasina guesses at only works if people really want to believe that truth exists – they'd rather go mad searching after the proof they are sure is there than admit that the proof never existed at all.
| Quote #2
Lady Croom: Mr. Hodge, ignorance should be like an empty vessel waiting to be filled at the well of truth – not a cabinet of vulgar curios. (1.1)
"The well of truth" again suggests that truth is something absolute and unchangeable: like water from a well, it just is. People are just passive receptacles in Lady Croom's metaphor, making truth something that comes into a person from outside, rather than something they have a part in creating.
| Quote #3
Valentine: When your Thomasina was doing maths it had been the same maths for a couple of thousand years. Classical. And for a century after Thomasina. Then maths left the real world behind, just like modern art, really. Nature was classical, maths was suddenly Picassos. But now nature is having the last laugh. The freaky stuff is turning out to be the mathematics of the natural world. (1.4)
This shift in mathematics seems almost like a shift in ideas of truth: instead of truth being above messy reality, truth becomes that which most accurately describes the mess.