这个看你考虑的是总的还是个人的，因为S=I是假定没有进出口的情况下所有saving都会被该模型内的人用于investment，这是一个假定了。如下原文：

Although the accounting identify S=I shows that saving and investment are equal for the economy as a whole, this does not have to be true for every individual household or firm. Larry's saving can be greater than his investment, and he can deposit the excess in a bank. Moe's saving can be less than his investment and he can borrow the shortfall from a bank. Banks and other financial institutes make these individual differences between saving and investment possible by allowing one person's saving to finance another person's investment.

多谢。
你的最后一句“Banks ...by allowing one person's saving to finance another person's investment ”.我想问的是，银行如果有100存款，只能贷出去80（准备金的关系），这样子储蓄无法都贷出去转化为投资，恒等式无法成立，你怎么看？

………

企业的非意愿存货投资

不是，哥们，这个模型没有准备金的概念，再者，不存在借不出去的情况，而是假定了全部saving都会被另一端的人借出用于购买new capitals(buildings, equipments or whatever)，即investment，这不是一个现实的情况，而只是一个模型，存在假定前提。