Deductive Reasoning

Reason can be casually defined as the making sense of things. For as many bad ideas that are out there bouncing around from generation to generation trying to make sense of this world, it is comforting to know that there are some basic rules that can help us understand the way things really are. There is a referee of sorts that keeps ideas in check if we are willing to listen. Reason is governed by the rules of logic.  Fundamentally, there are several types of reasoning: deductive, inductive, and abductive reasoning to name a few; but arguably the most powerful is deductive reasoning. All minds, both great and small, use deductive arguments as we go about our daily lives. Let’s look at the deductive form of reasoning so that we can better understand the structures that we use to think and solve problems.

Deductive reasoning is a kind of thinking that starts with a set of premises and moves towards a concrete conclusion. A set of premises that leads to a conclusion is also known as a syllogism. Deductive syllogisms are arguments that are logically airtight. This means that if the premises of the argument are true, then the conclusion must be necessarily true and there is no amount of reasoning that can change it. The most effective way to argue against a deductive argument is to question the premises of the argument.

For example, a deductive argument might be:

  1. If you are reading this post, then you are on the computer. 
  2. You are reading this post. 
  3. Therefore, you must be on the computer.

or symbolically,

  1. If A—>B
  2. A
  3. Therefore, B. 

Notice that this particular structure is a common valid argumentative form. (We will delve more into this form in the coming days). With this form, thinkers can mix and match any set of premises and, so long as they are true in the real world, the conclusions must be concretely true and can’t be false. In other words, if we accept that premise 1) and premise 2) are really true, then the conclusion must be true. As mentioned earlier, we can only debate the soundness (the real truthfulness) of premise 1) and 2). I would encourage you to think of arguments of your own following this form!

In conclusion, deductive reasoning has immense power and is one of the most commonly used types of reasoning. Because of this power, the usual course of debate shifts to the soundness of the argument’s premises. Therefore, when constructing a deductive argument, one must carefully consider and piece together his premises in such a way that they can withstand the test of a rational charge. One need not be a Roman philosopher to think deductively!


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