Deductive vs. Inductive

rn/writer, RN · Aug 30, 2009

Deductive reason starts with a rule or a law or a principle, then looks at specific examples to see if the idea applies to them. This is reasoning from the top (principle) down (to the example).

Inductive reason starts with specific examples and tries to see if a principle emerges. This is reasoning from the bottom (as many examples as possible) up (toward a ruling principle).

Deductive reasoning is favored by the sciences because one only has to see if an example "cooperates" with the particular principle to know if it is valid in the matter at hand. Inductive reason, based as it is on examples, is limited because it is impossible to try every scenario, so one always has to consider that there might be an exception somewhere along the way.

Here's a real-world application:

Deductive reasoning would start with the principle that gravity causes objects to fall to the ground. You let go of a shoe in midair, and, sure enough, it drops to the ground. Deductive reasoning tells you that the principle of gravity is at work.

Inductive reason would have you drop a shoe, a ball, a magazine, a spoon, and when they all fall to the ground, suspect that there is a force causing them to do so. You would, like Newton, surmise that gravity exists.

Hope this helps.

Wulfie · Aug 30, 2009

Thank you. So are these examples of deductive reasoning?:

My neighbor has been in the hospital for days

My neighbor is ill.

Because:

The hospital is for ill people.

My neighbor is in the hospital, therefore, my neighbor is ill

or:

Hypertension can kill you.

Hypertension may lead to kidney failure.

Hypertension may lead to Coronary Artery Disease.

Because:

Hypertension may lead to kidney failure.

Kidney failure may result in death

Hypertension may lead to Coronary Artery Disease. CAD may result in Death

therefore Hypertension can kill you.

I m still at a loss for inductive reasoning. Can you give me similar examples?

llg, PhD, RN · Aug 30, 2009

Inductive reason is how many people build theories. For example:

If I interview people and ask them about how they are coping with their new diagnosis of cancer -- and then use their responses to build a model about the common ways people cope -- I am using inductive reasoning. I am starting with specific cases and using them to develop general principles.

If I use a model of coping (apply general principles) to develop a treatment protocal for people newly diagnosed with cancer, I am using deductive reasoning. I am using general principles to make assumptions about what is needed in specific cases.

rn/writer, RN · Aug 30, 2009

Inductive reason would start from the examples and go to the rule.

I visited my uncle in the hospital. He was ill.

My neighbor was in the hospital. He was ill.

I was in the hospital a year ago. I was ill.

You might then surmise that the hospital is for people who are ill. But here's where you can get into trouble with inductive reasoning. Just because all the examples of people in the hospital that you can think of were people who were ill, that doesn't mean there aren't exceptions. Some people are in the hospital to have a baby. That isn't an illness. Some people are in the hospital because they have an injury that is not technically an illness.

Often, with inductive reasoning, what you are looking for are possibilities, not finite answers.

To use Ilg's example, you could use inductive reasoning to come up with possibilities for helping people cope with a cancer diagnosis, understanding that what works for one might not work as well for another. You aren't so much looking for a hard and fast rule as a range of things to choose from.