It is important to know how to design an argument that shows belief in its conclusion to be theoretically rational, and to know how to assess an argument that purports to do so. In addition to striving for theoretical rationality in our beliefs, however, we also frequently seek justifications for choices and actions. Many of those justifica-tions seek to establish that a choice or action is prudent or is a prac-tical undertaking, given our options and aims.The study of pracprac-tical reasoning attempts to clarify how we can use the notion of pru-dence effectively to guide our choices and actions.
Suppose you are trying to decide what to do when you don’t know what the future holds (surely a common situation!). You don’t
know what the weather is going to be like on the beach a month from now. Should you plan a vacation and risk spending a week inside doing jigsaw puzzles and watching daytime TV? This depends on how you weigh the chances of bad weather against how much you would like or dislike several of the possible consequences. According to the act/state/outcome (ASO) conception of decision making, you organ-ize your reasoning around the following ideas: First of all, distinguish the different acts that are available to you (for instance, taking or not taking a beach vacation). Second, consider the different states the world might be in (such as there being acceptable weather or there being lousy weather at the beach). Finally, decide how much you would value the outcomes of those acts given the different states the world might be in.You might even organize your thoughts by mak-ing a little diagram:
Reasoning:Truth and Prudence 23
Acts States
Lousy Weather Acceptable Weather Take a Beach Vacation lost time and money spend money; good time Don’t Take a Beach paid nothing; perhaps paid nothing; perhaps
Vacation some relief some regret
The two choices in the left column are the acts available, while the headings of the two columns on the right are the relevant possible states of the world.The remaining boxes are the outcomes of those acts given those two possible states.A given decision situation might involve many more relevant possible states of the world than just two, and many more relevant possible choices than two, but for simplicity let us start with this 2x2 diagram.
After filling the outcome boxes with qualitative information, you might try to describe that information quantitatively. As a first step, try to fill in that information by ordering the outcomes in terms of your preferences. Which outcome would you prefer? Supposing that 4 is your first choice, I assume that the top right outcome would get a 4;
the top left outcome would, by contrast, be the worst and so would get a 1; the bottom left seems better than the bottom right, and so the two get a 3 and a 2, respectively. Our table now looks like this:
We are almost done explaining practical rationality under condi-tions in which you don’t know everything about the world, but we have two more steps. Suppose you hear a reliable forecast that the weather at the beach is going to be excellent.That suggests that tak-ing a beach vacation is the rational thtak-ing to do. On the other hand, suppose you hear that a major tropical depression is on the way and that heavy rain is forecast for the entire week you had hoped to travel.
That suggests you should stay home. That is, how much you like or dislike various outcomes of your actions, given the various states of the world, needs to be balanced against the likelihood of those states.
That’s why it is not practically rational to buy a lottery ticket just because there is some chance that it will be a winning ticket.The ques-tion is how likely it is to be a winning ticket, and that likelihood needs to be weighed against the cash you lost in buying it. So let’s suppose that after consulting weather data for the area for the last two decades we conclude that the chance of acceptable weather at the beach is 75 percent.We can note this in our decision matrix at the top of each column. Furthermore, we can now derive a weighted sum of the different outcome values relative to the probabilities for different states of the world: just multiply each outcome value by the proba-bility of the relevant state of the world and then add across.The result is known as subjective expected utility, as illustrated below:
Acts States
Lousy Weather Acceptable Weather
Take a Beach Vacation 1 4
Don’t Take a Beach Vacation 3 2
Acts States
Lousy Weather OK Weather Expected Utility
(25%) (75%)
Take a Beach 1 x .25 = .25 4 x .75 = 3 .25 + 3 = 3.25 Vacation
Don’t Take a 3 x .25 = .75 2 x .75 = 1.5 .75 + 1.5 = 2.25 Beach Vacation
The subjective expected utility of taking the beach vacation is 3.25, while the subjective expected utility of not doing so is only 2.25, so from the point of view of practical rationality the former is more rational than the latter. In this case, take the vacation. The result would, of course, be different for different weather forecasts.
You might be concerned about the use I’ve made of the act/state/
outcome approach to practical rationality because it so radically sim-plifies both the real choices we have to make and the possible states of the world that face us. After all, there are lots of different ways in which I might take a beach vacation—I could travel by train or by car; I might do a lot of windsurfing or I might not; I might or might not take the dog. Similarly, there seem to be indefinitely many ways in which weather could be lousy or, by contrast, acceptable.
According to the ASO theory, however, the number of columns and rows you use to formulate a decision problem depends upon you. In particular, if various forms of lousy weather will make a difference for how you value effects of that the weather on your acts, then distin-guish different kinds of lousy weather in the decision matrix.
Otherwise, don’t bother. Similarly, if the different forms your vacation might take (with or without the family dog; windsurfing or not, etc.) make a difference for how much you value the possible outcomes, then distinguish those forms of vacation in the decision matrix.
Otherwise, don’t bother.
Now we are ready to say what it means to act rationally. Use, if you like, the act/state/outcome method to develop a decision matrix. Next, calculate, if you like, the subjective expected utility of the different acts available to you, making sure that your probabilities for the different possible states of the world always add up to exactly 1.Then, after your calculation, choose the act that has the highest subjective expected utility. If two or more acts are tied for first place in your subjective expected utility ranking, it doesn’t matter which one you choose so long as you choose one of them. As Yogi Berra once suggested, “If you come to a fork in the road, take it.”
Why did I use the parenthetical, “if you like” in my characteriza-tion of racharacteriza-tional decision? You could reasonably ask:“Do I have to use a decision matrix when deciding what to do?” To understand the answer to this question, notice that you often reason according to the rules of theoretical rationality without paying any attention to the fact that you’re doing so.That is, you can be theoretically rational without spelling out each of your lines of reasoning in terms of premises and a
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conclusion. So, too, the account we just gave of practical rationality doesn’t strictly require that every time you make a choice you have to write out a decision matrix and calculate subjective expected utilities.
As we’ll see in Chapter 4’s discussion of ethics, John Stuart Mill pointed out over a century ago that we often act rationally without performing any calculations. In fact, in some situations it would be positively irrational to calculate subjective expected utilities. (Can you think of one?) Some momentous or complex decisions do require that we calculate subjective expected utility, and the theory of practi-cal rationality gives us tools for doing that. Blaise Paspracti-cal’s famous wager is such an example, and we will discuss it with some care in the next chapter. In other cases, unless you are in the grip of habits that pos-itively violate the theory of practical rationality, the theory says you should do what comes naturally. As we’ll see in Chapter 5, that con-clusion is supported by recent work in experimental psychology.