Practical reasoning

The conclusion of the above argument about the automobile is one that has a practical conclusion: rather than saying that some proposition is true, it enjoins or commends a particular action. What the argument says, roughly, is that doing one thing (finding alternatives to the petrol-driven automobile) is necessary if a certain desirable outcome or end (finding alternatives to vehicles that emit carbon monoxide) is to be achieved. Other arguments with practical conclusions are those that say that a certain action would be sufficient to bring about the desired end, and for that reason ought to be performed. Still others say that a certain action would lead to a certain undesirable outcome, and therefore ought not to be performed. Reasoning of this kind is often called practical reasoning, or means-end reasoning, and is normally based upon two sorts of considerations: First, an outcome is specified as being either desirable or undesirable (in this book we prefer to use 'must' or 'should' to specify the outcome, as illustrated below); second, there is some proposition put forward that says either (1) that if such-and-such action is performed, the outcome will result; or (2) the outcome will result only if such-and-such action is performed (that is, the outcome will not come about unless the action is performed).

In fact, there are no fewer than eight different types of argument that concern a relation between action and outcome. Just for the purpose of illustration, then, suppose that the outcome is that the amount of chocolate in the world be increased, and the action in question is X. Then, using 'should', the eight types of practical reasoning can be sketched like this:

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