Philosophy of Wealth versus Economics

"For suppose that the person, whom we saw doubting between several little gratifications for himself, had thought after a while of a poor invalid whom he would pass on his way home; and had spent some time in making up his mind whether he would choose a physical gratification for himself, or would do a kindly act and rejoice in another's joy. As his desires turned now towards the one, now the other, there would be change in the quality of his mental states; and the philosopher is bound to study the nature of the change. But the economist studies mental states rather through their manifestations than in themselves; and if he finds they afford evenly balanced incentives to action, he treats them primâ facie as for his purpose equal."

Alfred Marshall, Principles of Economics