There is no cheery optimism in Aquinas with respect to reason. The human is disordered; one might even say we suffer a totality of depravity since not a single human capacity or function remains in the state of original justice. Yes, humans are utterly messed up, but they are still human beings, and as human beings, as rational animals, they still possess the natural law, for to lose the natural law would be a loss of humanity, actually to become a beast. Not, that is, to act bestially—humans do so—but to be a beast. And this has not happened, since original sin does not change our essence—nor could it. The basic human goods remain the same basic human goods for Adam and for Hitler, and the flourishing of human persons qua persons has not changed. But sin does change our willingness to function as we ought, as we can all attest.
There is, then, no contradiction between the natural law and original sin, at least as understood by Thomas Aquinas. The “Manhattan Declaration,” therefore, remains the declaration of cosmopolis, for insofar as the declaration is reasonable it is reasonable for all, even us sinners.
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