Economic theory states that at the end of the day, all direct costs (out-of-pocket costs) can be interpreted as opportunity costs. The former are the costs associated with doing a certain thing, the latter with not doing it. When for example we buy a shirt we will have an out-of-pocket cost corresponding to the price of the shirt and an opportunity cost equivalent to an article such as a polo shirt that we could have bought with that money instead of the shirt. This line of reasoning naturally presupposes a finite limit to our monetary resources at the time of the purchase (the so-called budget constraint) and in some cases also time limitations (in the sense of finding the time to make good use of the new item we have purchased). When we buy the shirt, for example, we will deploy monetary resources which will no longer be available for the purchase of the polo shirt. Moreover, at the time when we are using the shirt, we would not (normally) be able to simultaneously make use of the polo shirt.