This type of reasoning works exactly like it sounds like: you begin with the conclusion that you want and work backwards, looking for supporting logic and premises that support your argument. This is, of course, in no way rational. Most often, it is rationalizing, looking for justifications for when we are wrong.
For example:
Tammy and Jeff have set themselves the goal to save money for a vacation later in the year. This entails some sacrifice on both their parts, so they have agreed to cut back on spending on luxuries. Tammy is an avid action figure collector, and a new, pricey exclusive comes out on a figure that’s part of a series she’s a huge fan of.
She buys the figure, and anticipating Jeff’s fully warranted anger and disappointment thinks of an excuse that justifies her purchase. She comes up with the explanation that the entire set would be worth much less without that particular piece and that it is in fact an investment and not a luxury.
The objective can also be exclusively to convince one’s opponent with no interest in what is the actual truth – points that support the conclusion are sought out to the exclusion of any that contradict it. This type of reasoning also has the pitfall that one could end up finding no justification at all, winding up having to resort to logical fallacies to make up for the paucity of the arguments. It should not be relied upon as a source of vindication – rather look for a superior method for the argument to be constructed.
Backwards reasoning does have some practical uses, though. It can act as a good starting point for finding superior methods of reasoning to use – start from the outcome, work backwards and you may find a valid and satisfactory train of logic on which to base your actual argument.
Another positive use of this type of reasoning is when planning for a future outcome. You begin with an objective that you wish to achieve at some point in the future and enumerate the step that must come before that, and the step before that, and so on, until you reach the present. In this manner, you can make concrete plans and use that analysis to convince someone.
This is a type of cause-and-effect reasoning, but can be in some ways superior to the usual method of thinking forward in time. A single cause can have a great many effects, but in general, an effect will have a single cause, or very few potential causes.
