Opportunity Cost
The cost of a choice includes the best alternative given up, not only money spent.
The Mechanism: Opportunity cost is the value of the best alternative forgone when a choice is made. It includes time, attention, and resources that could have been used elsewhere, not just direct financial expense.
The Philosophical Pivot: A decision's cost is personal and contextual. Two people can make the same purchase while giving up very different opportunities because their circumstances differ.
The Horizon: The concept helps compare investments, career choices, and public spending. It cannot provide a single answer unless the alternatives and their values are made explicit.
Sunk costs are already spent and generally should not determine a future choice, while opportunity costs concern the best available path from now onward.
Not every meaningful value can be reduced to a common unit. A framework can clarify tradeoffs without making them morally interchangeable.
Seeing the forgone alternative can turn an automatic decision into a deliberate one.
Every yes spends something, even when the thing spent is never named.
What is the best alternative I am giving up, and would I still choose this if that cost were visible?