Induction, or inductive reasoning, proceeds from particular observations to a generalized conclusion. If we notice that one houseplant grows better in direct sunlight, we might use induction to conclude: Houseplants grow better in direct sunlight. If we then place a second houseplant in direct sunlight and it also grows better, we would be more certain of our conclusion. The more instances we observe of a houseplants thriving in direct sunlight, the more likely our conclusion is to be correct. However, inductive reasoning only leads us to a conclusion that is probably, not certainly, true. We would only have to place a fern in direct sunlight to see it wither and find out that not all houseplants grow better in direct sunlight.
This flaw of induction was most notably pointed out by Scottish philosopher David Hume, who concluded that that from no finite number of observations could a unrestrictedly general conclusion be drawn. This principle was highlighted when the schoolroom adage “All swans are white” was proved false by the discovery of black swans in Australia in the 18th century.
Deduction, or deductive reasoning, starts with a general principle and makes a specific conclusion based on the circumstances. As long as the general principle and circumstantial particulars are correct, deduction leads to a conclusion that is always correct. Here is a simple example of deductive reasoning: Given that a laptop commuter needs either a charged battery or an external power source to operate, and given a particular computer that is neither plugged in or containing a charged battery, we can conclude that the laptop computer will not operate.
We use a combination of inductive and deductive reasoning in our everyday lives. If we get into a 20 mpg, 10-gallon car and discover that if has only 1/4 of a tank, and we have to drive 60 miles, we can conclude, through the use of mathematics, that we will not be able to make it to our destination. (10 gallons x 1/4 x 20 miles/gallon = 50 miles.) That is deductive reasoning. We know that we will not make it 60 miles by the laws of physics. If we fill up the tank and then decide not to take the freeway because the freeway has been crowded the last three mornings, we are using inductive reasoning. We are making a conclusion based on past experiences. The freeway will probably be crowded again in the morning, but it possibly may not.
Intuition is yet another form or reasoning. It is based on a feeling or a hunch. Often, however, intuition is based on inductive reasoning that we are either not conscious of or that is too complex to explain. Intuition is often wrong, and as there is no way to examine the intuitive process, we are at a loss for ways to improve it.