“That is true liberty, which bears a pure and firm breast.”

By pure, I'm assuming he meant at natural law: morality, integrity, rectitude, and a forthright candor. And, by firm breast, I'm assuming he meant the courage, fortitude and ability to stand in such.

The corruption of the 'heart' leads to innumerable paths of deceit, delusion, and dependence. One cannot walk straight if leaning to the right or the left.