Utility is a unit of accounting that works much better than money. It's actually called a "utility function" because the utility varies depending on the inputs,

Utility is a unit of accounting that works much better than money. It's actually called a "utility function" because the utility varies depending on the inputs, including time, number of units, etc. For example, let's say you've always wanted to go skydiving. You pay $500 and go skydiving. It's a great value to you. But when you get back to the ground, would you pay $500 to do it again? Probably not, since the utility of going skydiving has dropped for you (because you crossed it off your bucket list), even though the price has stayed the same. Now imagine the price for skydiving has dropped to $25. You might go again. Heck, you might go every day for a week. But after a while, the novelty wears off, and you won't go anymore. Even if the price drops to free. In fact, if someone was forcing you to go skydiving every day, you might even PAY to do something else instead. On the other hand, maybe you need to skydive every day because you are trying to qualify as a stuntman. That means the utility of being qualified is greater than the negative utility of "boredom from too much skydiving". Or maybe you NEVER go skydiving because you went base jumping several years prior, and skydiving seems similar enough that it's just not worth $500 to you. Trying to representing these trade-offs with only money is confusing and/or impossible. Try to work out the "boredom" example using only money: Someone can know they get more utility from X than Y, but not be able to express prices for X and Y.