If, for example, you give or consider reasons why clearcutting ought to be legal (or not legal), then you are engaging in prescriptive reasoning. Likewise, if you argue for a certain economic policy, or try to convince a friend not to drive so much, or reason through alternative life plans, you are also doing prescriptive reasoning.

The variety of forms of prescriptive reasoning is endless, but there are some fairly common types which you should learn to recognize and use. The two most common types are rule-based reasoning and consequentialist reasoning.

Rule-based reasoning is characteristic of the deontological tradition in ethics. The deontological tradition seeks to find moral laws that can be used to reach moral decisions in specific cases. Thus rule-based reasoning employs one or more general moral rules as premises, together, usually, with some additional premises that describe the relevant facts of the situation. Here's an example:

         It is wrong to break a promise simply because you preferred to do something else.
         James promised his son that he would take him fishing today.
         But when it came time to go, James decided to stay home and watch TV instead.
         The only reason James stayed home was that he preferred to watch TV.
So     It was wrong of James not to take his son fishing, as he had promised.

Here is another example of rule-based moral reasoning, but in this case the rule is not one that is widely accepted:

        All killing is wrong.
        Slaughtering animals is killing.
So    Slaughtering animals is wrong.

Because moral rules are frequently controversial, it might seem desirable to avoid them altogether. But without moral premises we can not validly derive a moral conclusion. Philosophers often express this by saying that you can't derive "ought" from "is", meaning that if your premises assert only facts (what "is") and not moral principles (what "ought" to be), you can't validly infer a moral conclusion (an "ought").

We do not, however, always base our moral reasoning on rules. Particularly when we are dealing with public policy, where there is likely to be a diversity of moral opinion, it is often useful to reason consequentally. Consequentialist reasoning bases its moral conclusions on the consequences or effects of various proposed courses of action. It is the kind of reasoning most often assoiciated with the utilitarian tradition in ethics. Consequentialist reasoning lays out the various options, evaluates each in terms of its consequences, and then draws a moral conclusion from that evaluation -- assuming that we ought to do what is best or avoid what is worst. Here is an example:

1.         We can either add chlorine to our public drinking water or not; these are our only options.
2.         The major consequences of adding chlorine are that we will have a slightly higher cancer rate, due to the chlorinated compounds in the water, but we will prevent a great many deaths byinfectious disease.
3.         Conversely, the major consequences of not adding chlorine are that we will have a slightly lower cancer rate but a much higher rate ofdeath by infectious disease.
4.         It is better to have a slightly higher cancer rate than to have a much higher rate of death by infectious disease.
5.         (When faced with a choice of actions, we should choose the one with the best consequences)
6. So    We should add chlorine to our public drinking water.

1. A listing of all the reasonable options for action: It is important to consider all reasonable options; if you ignore one, it might be the one that should have been chosen. In the argument above, the first premise lists the options. One reasonable question we might raise about this premise is whether there might be something else we could add to the water supply that would not be carcinogenic and yet would prevent infectious disease.
2. A listing of all the important consequences of each option:  In our example, this is done in the second and third premises. If important consequences are ignored, then the argument can be refuted by pointing them out. I think that I've included the important consequences in this argument, but (not being an expert on the subject) I could be wrong.
3. A judgment concerning which set of consequences is best (or least bad): In the argument above, this judgment is expressed in the fourth premise. Here the judgment seems obviously right, but in many cases it will need to be supported by additional premises.
4. An assertion of the principle that we should choose the option with the best (or least bad) consquences: This is the fundamental moral principle underlying all consequentialist reasoning.  It is expressed in the fifth premise of our sample argument. It is so fundamental that it is almost never explictly stated. In a real argument, it would typically be an implicit (unstated) premise, which is why I have put it in parentheses.
5. The conclusion: The conclusion, of course, will be that we should do what was determined in the premises to have the best consequences.

Consequentialist reasoning in the form I have presented it here is valid. Rational debate, then, must center on whether or not the premises are true in a particular application. To criticize consequentialist reasoning which is presented in this form, you must raise doubts about the truth of one or more of the premises (the first four statements).