Text: Inductive and Deductive Arguments


In the process of deduction, you begin with some statements, called “premises,” that are assumed to be true, you then determine what else would have to be true if the premises are true.

For example, you can begin by assuming that God exists, and is good, and then determine what would logically follow from such an assumption. You can begin by assuming that if you think, then you must exist, and work from there.

With deduction you can provide absolute proof of your conclusions, given that your premises are correct. The premises themselves, however, remain unproven and unprovable.[1]

Examples of deductive logic:

  • All men are mortal. Joe is a man. Therefore Joe is mortal. If the first two statements are true, then the conclusion must be true.[2]
  • Bachelors are unmarried men. Bill is unmarried. Therefore, Bill is a bachelor.[3]
  • To get a Bachelor’s degree at Utah Sate University, a student must have 120 credits. Sally has more than 130 credits. Therefore, Sally has a bachelor’s degree.

Two rectangles. Left: General Principle. Right: Special Case. An arrow pointing left to right above them is labeled "deductive reasoning." An arrow pointing right to left below them is labeled "inductive reasoning."Induction

In the process of induction, you begin with some data, and then determine what general conclusion(s) can logically be derived from those data. In other words, you determine what theory or theories could explain the data.

For example, you note that the probability of becoming schizophrenic is greatly increased if at least one parent is schizophrenic, and from that you conclude that schizophrenia may be inherited. That is certainly a reasonable hypothesis given the data.

However, induction does not prove that the theory is correct. There are often alternative theories that are also supported by the data. For example, the behavior of the schizophrenic parent may cause the child to be schizophrenic, not the genes.

What is important in induction is that the theory does indeed offer a logical explanation of the data. To conclude that the parents have no effect on the schizophrenia of the children is not supportable given the data, and would not be a logical conclusion.[4]

Examples of inductive logic:

  • This cat is black. That cat is black. A third cat is black. Therefore all cats are are black.[5]
  • This marble from the bag is black. That marble from the bag is black. A third marble from the bag is black. Therefore all the marbles in the bag black.[6]
  • Two-thirds of my latino neighbors are illegal immigrants. Therefore, two-thirds of latino immigrants come illegally.
  • Most universities and colleges in Utah ban alcohol from campus. That most universities and colleges in the U.S. ban alcohol from campus.

Deduction and induction by themselves are inadequate to make a compelling argument. While deduction gives absolute proof, it never makes contact with the real world, there is no place for observation or experimentation, and no way to test the validity of the premises. And, while induction is driven by observation, it never approaches actual proof of a theory. Therefore an effective paper will include both types of logic.[7]