MAMUHUNAN WAS INVITED BY HIS FRIENDS TO INVEST IN A CORP., A NEWLY ORGANIZED FIRM ENGAGED IN MONEY MARKET FINANCING OPERATION. BECAUSE OF HIS HEAVY INVESTMENTS, MAMUHUNAN BECAME THE FIRM'S PRESIDENT AND, AS SUCH, PURCHASED A BIG NUMBER OF COMPUTERS, TYPEWRITERS AND OTHER EQUIPMENT FROM TAKTAK CORP. ON INSTALLMENT BASIS. A CORP. PAID THE DOWN PAYMENT AND TAKTAK CORP. ISSUED THE CORRESPONDING RECEIPT. TO HIS CHAGRIN, MAMUHUNAN DISCOVERED THAT THE ARTICLES OF INCORPORATION HAD NOT BEEN FILED BY HIS FRIENDS ON THAT DATE SO HE HURRIEDLY ATTENDED TO THE MATTER. NO SOONER HAD THE CERTIFICATE OF INCORPORATION BEEN ISSUED BY THE SEC; A CORP. BECAME BANKRUPT AFTER THREE MONTHS. UPON BEING SUED BY TAKTAK CORP. IN HIS PERSONAL CAPACITY, MAMUHUNAN RAISED AMONG ITS DEFENSES THE DOCTRINES OF DE FACTO CORPORATIONS AND CORPORATIONS BY ESTOPPEL. CAN THE TWO DEFENSES BE VALIDLY RAISED?