Wills Without Witnesses
Pennsylvania is the only state that does not require written wills to have any subscribing witnesses in order to be valid.

When a will is presented to the Register of Wills for probate, it must be "proven." A will is proven when two people appear before the Register of Wills to sign an affidavit stating that they are familiar with the testator's signature and that the signature on the will belongs to the testator.  Even wills that have two subscribing witnesses must be proven. Having subscribing witnesses merely provides two people that can be looked to first to do so.

In addition to providing two potential candidates to prove the will, subscribing witnesses also provide other advantages that must be considered.

See: Witnesses, Advantages; Self-Proving Affidavit