The deductive method is an approach to reasoning that is based on deduction, or starting from a general case and, from that general case, drawing a conclusion about something more specific. An argument based on this method may be formulated as such: "All men lie. Dave is a man, therefore Dave lies." Of course, the rightness or wrongness of the specific conclusion is entirely reliant on the correctness of the general claim; if the general claim is wrong, specific conclusions deduced from it are also wrong, or at the very least are incorrectly deduced. In contrast to deductive reasoning, inductive reasoning involves starting from specific cases and, from them, drawing a general conclusion.
Reasoning used in the deductive method can be presented, formally or informally, in a variety of different ways. One of the most common forms of the deductive method is the syllogism, in which two conditional statements are given and from them a conclusion is drawn. For example, a syllogism can take the following form: "If Dave is late for work again, his boss will be angry. If Dave's boss is angry, Dave will not get a raise. Therefore, if Dave is late for work again, Dave will not get a raise." The conclusion that Dave will not get a raise if he is late for work is drawn from the two preceding conditional statements.
Much crime-solving fiction, most notably the Sherlock Holmes stories, is based in the deductive method. In such contexts, this method is a process for solving crimes based on the application of deductive reasoning to criminal cases. A detective may apply some general knowledge about criminal psychology or crime scene investigation to the specifics of the case at hand in order to draw conclusions about the identity and methods of the criminal. The deductive method is actually used in many real-life crime solving situations as well, as many methods of investigation are based in the application of general knowledge applied to specific cases.
Reasoning in day-to-day life, both in professional and personal contexts, is often based on the deductive method. Medical examination, investigation, and treatment, for instance, are all based on the application of general medical knowledge to specific individual patients. Much research in fields as diverse as economics, physics, and biology make use of both deductive and inductive reasoning. Researchers make hypotheses and explain some results based on general rules but often need to make new general theories to explain specific results that do not fit into the existing theoretical framework.