Everybody knows that the same sum of money is of much greater value to a poor man that to a rich one. Give £10 a year to the man who has but £10 a year, you double his income, and you nearly double his enjoyments. Add £10 more, you do not add to his enjoyments so much as you did by the first £10. The third £10 is less valuable than the second, and the fourth less valuable than the third. To the possessor of £1,000 a year the addition of £10 would be scarcely perceptible; to the possessor of £10,000 it would not be worth slooping for.
The richer a man is the less he is benefited by any further addition to his income. The man of £4,000 a year has four times the income of the man who has but £1,000; but does anybody suppose that he has four times the happiness?
John Stuart Mill, ‘Primogeniture’, in The Collected Works of John Stuart Mill, Toronto, 1988, vol. 26, p. 336