Inductive and Deductive Syllogisms
by Harvey Bluedorn
The labels inductive and deductive may be applied to several things, including methods of reasoning and methods of studying. A deductive approach moves from the rule to the example, and an inductive approach moves from the example to the rule.
I will first discuss deductive and inductive methods of reasoning (I have discussed these in greater detail elsewhere), then I will discuss deductive and inductive methods for studying.
The Deductive Method of Reasoning
The deductive method reasons from certain premises to a necessary conclusion. It is often described as reasoning from the general to the specific.
Premise: All men are mortal.
Premise: Socrates is a man.
Conclusion: Socrates is mortal.
If the premises are true, and the form is correct or valid, then the conclusion is necessarily true. However, if the form is invalid, then the conclusion is not necessarily true.
Some men are mortal.
Socrates is a man.
Therefore Socrates is mortal.
Though we may know that Socrates is mortal, nevertheless that does not logically flow from the premises of this argument. If we only know that some men are mortal, then Socrates might be among some men who are not mortal. The form of the argument is not valid.
The Inductive Method of Reasoning
The inductive method reasons in the opposite direction of the deductive method. It begins with specific observations and reasons to a generalization about the observations. It is often described as reasoning from the particulars to the general.
I have examined ten thousand dogs.
Every dog I have examined has fleas.
Therefore, all dogs have fleas.
The conclusion (really, a generalization) may possibly be true there is no observation which contradicts the conclusion but it is not necessarily true there are still more observations which could be made.
If, indeed, I had examined all dogs (which, of course, nobody could possibly do), and all dogs examined had fleas, then I could conclude that all dogs do indeed have fleas. Based on my sample of dogs, it appears that all dogs have fleas. But the first dog I found which did not have fleas would contradict and therefore disprove my conclusion. So all that I actually know is that some dogs have fleas.
A Comparison of Inductive and Deductive Methods of Reasoning
The deductive method of reasoning moves toward necessary conclusions derived from correct connections between premises premises which are all either given or assumed to be true.
The inductive method of reasoning moves toward possible conclusions derived from hypothetical connections between premises (observations) which are selected from among all possible true premises (observations).
Ideally, the deductive method of reasoning is objective in its conclusions (the conclusions are necessarily true), but subjective in its premises (the premises are assumed to be true).
Ideally, the inductive method of reasoning is subjective in its conclusions (the conclusions are not necessarily true), but objective in its premises (the premises are observed to be true).