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There are two basic ways of gaining information about the world: inductive reasoning and deductive reasoning. Inductive reasoning starts by looking at information and trying to figure out the causes; deductive reasoning takes starting assumptions and derives things from them.
 Inductive reasoning
Inductive reasoning is a means of gaining information about reality by starting with results, outcomes, or effects, and then inferring the causes.
- Experimental science obtains data and evaluates competing hypotheses or explanations for how the data came about.
- Medicine uses symptoms presented by patients to diagnose disease.
- Any logical process of the form "If outcome Y, then possible cause(s) X".
Medicine is a good example of how induction differs from deduction. In diagnostic medicine, patients generally do not walk into hospitals and say, "I have disease X. Please tell me what symptoms Y I should have." If they did, doctors could always reason deductively to find the symptoms Y for disease X. (Recall that "if X, then Y" is deduction.) Instead, patients say "I have symptoms Y. What disease X do I have?"
Inductive reasoning has the advantage of being flexible, and has the ability to evaluate competing hypotheses even when information incompatible with the known causes or hypotheses is observed.
The disadvantage of induction is that absolute truth and objectivity are compromised. Outcomes Y could have several different causes X. For instance, a patient may exhibit a sore throat, a runny nose, a rash, and splenomegaly. There may be several different diseases that cause those symptoms (e.g., acute retroviral syndrome or mononucleosis); the hard task of figuring out which one to diagnose is the reason doctors make big bucks. Furthermore, the diagnosis of one doctor may not be the same as another (although only one of them may be correct); hence, there is a subjective component to induction and the need for the use of evidence.
David Hume argued that inductive reasoning cannot be rationally justified at all, and that whenever we make inductive inferences, we rely on the 'uniformity of nature', the assumption that the universe is uniform and does not change.
 Deductive reasoning
Before the advent of inductive reasoning in science, knowledge was mainly gained in the more abstract deductive reasoning, which begins with assumptions, axioms, or hypotheses, and then makes derivations from them. This is still used in mathematics, logic, and philosophy, and up to around the 17th century, in the sciences (hence things like "natural philosophy", i.e., physical sciences).
Deductive reasoning has the advantages of certainty and objectivity. The statement "If X, then Y" guarantees that Y is true when X is true, regardless of belief. The question then becomes "Is X true?", which is opt to be challenged for near anything.