Private autonomy encompasses both contractual autonomy—the ability to engage freely in contractual relationships—and a broader freedom to regulate all legal acts concerning one's juridical sphere. While testamentary dispositions are a significant expression of private autonomy, this freedom is not absolute. Conditions that undermine fundamental rights, such as the right to convert or adhere to a particular religion, are considered contrary to public order and human dignity. Art. 634 of the Italian Civil Code renders conditions impossible to fulfill or contrary to imperative norms, public order, or good morals as null and void. The "favor testamenti" principle ensures that such unlawful conditions are treated as non-existent unless they are the sole motivation for the disposition, in which case Article 626 applies, nullifying the entire provision. The interplay between private autonomy and constitutional values, particularly human dignity and freedom of religion, buttresses the illegality of conditions restricting religious conversion in testamentary dispositions. These conditions conflict with the broader instrumental role of private autonomy in safeguarding economic initiative and the free enjoyment of property (Articles 41 and 42 of the Constitution). Thus, any testamentary clause limiting an heir's freedom to convert or adhere to a religion contravenes not only public policy but also the constitutional guarantee of personal dignity and religious freedom, rendering such clauses unenforceable.