Title I — ORGANIZATION AND FUNCTION OF THE BUREAU OF INTERNAL REVENUE SEC. 6.  Power of the Commissioner to Make Assessments and Prescribe Additional Requirements for Tax Administration and Enforcement. — (A) Examination of Returns and Determination of Tax Due. — After a return has been filed as required under the provisions of this Code, the Commissioner or his duly authorized representative may authorize the examination of any taxpayer and the assessment of the correct amount of tax, notwithstanding any law requiring the prior authorization of any government agency or instrumentality:2 Provided, however, That failure to file a return shall not prevent the Commissioner from authorizing the examination of any taxpayer. The tax or any deficiency tax so assessed shall be paid upon notice and demand from the Commissioner or from his duly authorized representative. Any return, statement or declaration filed in any office authorized to receive the same shall not be withdrawn: Provided, That within three (3) years from the date of such filing, the same may be modified, changed, or amended: Provided, further, That no notice for audit or investigation of such return, statement or declaration has, in the meantime, been actually served upon the taxpayer. (B) Failure to Submit Required Returns, Statements, Reports and other Documents. — When a report required by law as a basis for the assessment of any national internal revenue tax shall not be forthcoming within the time fixed by laws or rules and regulations or when there is reason to believe that any such report is false, incomplete or erroneous, the Commissioner shall assess the proper tax on the best evidence obtainable. In case a person fails to file a required return or other document at the time prescribed by law, or willfully or otherwise files a false or fraudulent return or other document, the Commissioner shall make or amend the return from his own knowledge and from such information as he can obtain through testimony or otherwise, which shall be prima facie correct and sufficient for all legal purposes. (C) 2 Authority to Conduct Inventory-taking, Surveillance and to Prescribe Presumptive Gross Sales and Receipts. — The Commissioner may, at any time during the taxable year, order inventory-taking of goods of any taxpayer as a basis for determining his internal revenue tax liabilities, or may place the business operations of any person, natural or juridical, under observation or surveillance if there is reason to believe Introduced by Section 4 of the TRAIN Law. National Internal Revenue Code of 1997 (as amended by the TRAIN Law, RAs 11256, 11346 and 11467) 3