not even Ricardo could figure that out,

甚至李嘉图都没有想出来
this equation marginal utility, wait until 1871.

边际效用等式直到1871年才被提出
And again, to repeat it, it's of course very obvious now

现在看起来很理所当然
but wasn't at the time,

当时却不是
how can you describe what these people are doing?

你怎么能够描述人们在做什么呢
You have to figure out the budget constraint,

你得指出预算限制
that's what they can afford,

这是他们所能支付的
and then they're going to choose the point on their budget constraint

然后他们从预算限制中找那个点
which maximizes their utility.

那个点最大化他们的效用
But that just means in the picture

但这只意味着在图里
it makes the indifference curve tangent to the budget set,

让无差异曲线和预算线相切
which means that you set

也就意味着你来设定