Philosophical propositions can be supported—but differently

Take:

“All human beings possess equal moral worth.”

That is a philosophical proposition.

Historical evidence alone cannot establish it in the same way that historical evidence can establish:

“The Thirteenth Amendment abolished slavery.”

The latter can be demonstrated directly through constitutional and historical evidence.

The first requires arguments from moral philosophy.

For example:

Kantian argument

Human beings possess intrinsic moral worth and therefore should not be treated merely as means.

Utilitarian argument

Equality may be defended because equal consideration of interests produces better moral outcomes.

Natural-rights argument

All human beings possess certain rights by virtue of being human.

So philosophical propositions can absolutely be defended, but their justification is not simply:

“Here is a historical document proving it.”