• Bill Gates, Nicola Tesla, Leonardo Da Vinci, Barack Obama, Bill Clinton, Abraham Lincoln, James Cameroon, Amitabh Bachchan and many other famous personalities are/were left handed. If you are left handed you will be famous.
• John is a great Doctor and I am sure his son will also be a great Doctor too.
• Maya, Anita and Mary watch lot of movies and they tell me that PK is not a good movie. Therefore PK is a bad movie and I should not watch that movie.

We often make or hear such conclusions in our daily conversations. How do you make sense of these conclusions and is there a way to validate or rebut such conclusions. Ancient Philosophers established two main types of reasoning to test the validity of such observations and construct rationale arguments. They are Inductive Reasoning and Deductive Reasoning. Today this approach is extensively used in scientific methods, psychology and in many other fields.

Inductive Reasoning

In short, Inductive Reasoning is drawing conclusions (forming theories) based on specific facts and patterns of observation. We see some events and through heuristics, we form some models or concepts within our own mind to explain the event.

Inductive Reasoning can be strong or weak depending upon the premise and other observations that hold up the conclusion. Let us evaluate the following two examples of Inductive Reasoning.

• All the rivers I crossed flow toward the ocean. Therefore, all the rivers are flowing toward the ocean – This Induction is true for all rivers hence this is a strong Induction
• Month of August has experienced drought for the last ten years. Therefore, there will be drought conditions here for every August in future – This Induction may hold true or may not and hence it is a weak Induction.

The appearance of a black cat predicts misery, hunger and drought. What kind of Induction is this?

Deductive Reasoning

Deductive Reasoning moves in the opposite direction. It draws conclusions based on established theories and hypothesis.

Again Deductive Reasoning can be strong or weak depending upon the premise and the validity of the theory. Let is evaluate the following two examples.

• All oranges are fruits; All fruits grow on trees; Therefore, all oranges grow on trees – This is a valid and strong Deduction.
• All flight attendants know how to swim; Ralph knows how to swim; Hence, Ralph is a flight attendant – This is a weak Deduction and an untrue conclusion. It is not necessary that only flight attendants know how to swim. Anybody can swim.

If you are into literature, ‘The Science of Deduction’ – Does that ring any bell? Here is a quote from ‘A Study in Scarlet’ –

“In solving a problem of this sort, the grand thing is to be able to reason backwards. That is a very useful accomplishment, and a very easy one, but people do not practice it much. In the every-day affairs of life it is more useful to reason forwards, and so the other comes to be neglected. There are fifty who can reason synthetically for one who can reason analytically…Let me see if I can make it clearer. Most people, if you describe a train of events to them, will tell you what the result would be. They can put those events together in their minds, and argue from them that something will come to pass. There are few people, however, who, if you told them a result, would be able to evolve from their own inner consciousness what the steps were which led up to that result. This power is what I mean when I talk of reasoning backwards, or analytically.” –  Sherlock Holmes Quote

The understanding of the two types of reasoning will help us make a better sense of the world around us and perhaps test some of the long held beliefs/superstitions.