In National Union of Workers in Hotels, Restaurants and Allied Industries-Manila Pavilion Hotel Chapter v. National Labor Relations Commission,[41] we held that " [t]he burden of proof rests upon the party who asserts the affirmative of an issue. And in labor cases, the quantum of proof necessary is substantial evidence, or such amount of relevant evidence which a reasonable mind might accept as adequate to justify a conclusion."[42] What is indubitable in this case is that petitioner alleged in his Position Paper that there was a CBA with AMOSUP (a local union of which he was purportedly a member) which entitled him to disability benefits in the amount of US$80,000.00. It is elementary that petitioner had the duty to prove by substantial evidence his own positive assertions. He did not discharge this burden of proof when he submitted photocopied portions of a different CBA with a different union. In all, we find that the Court of Appeals committed no error in ruling that the Labor Arbiter's award of US$80,000.00 in disability benefits was unsupported by the evidence on record, even if we take into consideration petitioner's late documentary submissions. There is no cogent reason to disturb the appellate court's finding that the only credible and competent bases for an award of disability benefits to petitioner are the POEA Standard Contract of Employment and petitioner's own medical evidence that his disability grade is Grade 11 (14.93%). Thus, the Court of Appeals' computation of petitioner's permanent medical unfitness benefits in the amount of US$7,465.00[43] must stand. WHEREFORE, the instant petition for review is DENIED. The Decision dated May 31, 2005 and the Resolution dated July 14, 2005 of the Court of Appeals in CA-G.R. SP No. 82638 are AFFIRMED. No pronouncement as to costs. SO ORDERED. Corona, C.J., (Chairperson), Velasco, Jr., Del Castillo, and Perez, JJ., concur. [1] Rollo, pp. 152-163; penned by then Court of Appeals Associate Justice Jose Catral Mendoza (now a member of this Court) with Presiding Justice Romeo A. Brawner and Associate Justice Edgardo P. Cruz, concurring. [2] Id. at 187-188. [3] Id. at 87-96. [4] Id. at 29. [5] CA rollo, pp. 41-42. [6] Id. at 43.