A person commits theft when:
I. He obtains property of another which he knows to have been lost or mislaid, or to have been delivered under a mistake as to the identity of the recipient or as to the nature or amount of the property, without taking reasonable measures to return the same to the owner, and
II. He has the purpose to deprive the owner of such property when he obtains the property or at any time prior to taking the measures designated in paragraph I.