A person is disabled, and thereby eligible for Social Security disability insurance benefits and Supplemental Security Income (SSI), "only if his physical or mental impairment or impairments are of such severity that he is not only unable to do his previous work but cannot, considering his age, education, and work experience, engage in any other kind of substantial gainful work which exists in the national economy." 42 U. S. C. §§423(d)(2)(A), 1382c(a)(3)(B) (emphasis added) (hereinafter §423(d)(2)(A)). After her job as an elevator operator was eliminated, respondent Thomas applied for disability insurance benefits and SSI. An Administrative Law Judge (ALJ) found that her impairments did not prevent her from performing her past relevant work as an elevator operator, rejecting her argument that she is unable to do that work because it no longer exists in significant numbers in the national economy. The District Court affirmed the ALJ, concluding that whether Thomass old job exists is irrelevant under the Social Se