The Social Security Act authorizes payment of disability insurance benefits and Supplemental Security Income to individuals with disabilities. The Act defines the key term 'disability' as an 'inability to engage in any substantial gainful activity by reason of any medically determinable physical or mental impairment which can be expected to result in death or which has lasted or can be expected to last for a continuous period of not less than 12 months.' Walton (P) applied for Title II disability insurance benefits and Title XVI Supplemental Security Income. P had developed a serious mental illness involving schizophrenia and associated depression. P had lost his job as a full-time teacher but by mid-1995 he began to work again part-time as a cashier by December 1995 he was working as a cashier full time. The Agency (D) concluded that P's mental illness had prevented him from engaging in any significant work, i.e., from 'engaging in any substantial gainful activity,' for 11 months-from October 31, 1994 (when he lost his teaching job) until the end of September 1995 (when he earned income sufficient to rise to the level of 'substantial gainful activity'). P was not entitled to benefits because his disability lasted only 11 months, not the required 12. The District Court affirmed the Agency's decision, but the Court of Appeals reversed. The court said that the statute's 12-month duration requirement modifies the word 'impairment,' not the word 'inability.' The Fourth Circuit concluded P became 'entitled' to Title II benefits no later than April 1995, five months after the onset of his illness. The Supreme Court granted certiorari.