So I dug up a philosophy gem from a lesser known philosopher, Jeremy Bentham, dating back nearly 200 years, which illustrates what an economist today might call the diminishing marginal utility of wealth:
So far as depends on wealth, -- of two persons having unequal fortunes, he who has most wealth must by a legislator be regarded as having most happiness.
But the quantity of happiness will not go on increasing in anything near the same proportion as the quantity of wealth...
The effect of wealth in the production of happiness goes on diminishing, as the quantity by which the wealth of one man exceeds that of another goes on increasing: in other words, the quantity of happiness produced by a particle of wealth (each particle being of the same magnitude) will be less and less at every particle; the second will produce less than the first, the third than the second, and so on. ~ Jeremy Bentham (1748 - 1832), 'Pannamonial Fragments', Works, III, p. 228