Deductive Reasoning

Some would argue deductive reasoning is an important life skill.

It allows you to take information from two or more statements and draw a logically sound conclusion.

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Deductive reasoning moves from generalities to specific conclusions. Perhaps the biggest stipulation is that the statements upon which the conclusion is drawn need to be true.

If they’re accurate, then the conclusion stands to be sound and accurate. Let’s explore some deductive reasoning examples. See if you would’ve drawn the same conclusions yourself.

What Is Deductive Reasoning?

Deductive reasoning is a type of deduction used in science and in life. It is when you take two true statements, or premises, to form a conclusion. For example, A is equal to B. B is also equal to C. Given those two statements, you can conclude A is equal to C using deductive reasoning.

Now, let’s look at a real-life example.

1. All dolphins are mammals.
2. All mammals have kidneys.

Using deductive reasoning, you can conclude that all dolphins have kidneys. Remember, for this to work, both statements must be true. Okay, now that you have a good grasp on it, try a few examples.

Examples of Deductive Reasoning

Everyday life often tests our powers of deductive reasoning. Did you ever wonder when you’d need what you learned in algebra class?

Well, if nothing else, those lessons were meant to stretch our powers of deductive reasoning. Remember, if a = b and b = c, then a = c. Let’s flesh that out with added examples:

• All numbers ending in 0 or 5 are divisible by 5. The number 35 ends with a 5, so it must be divisible by 5.
• All birds have feathers. All robins are birds. Therefore, robins have feathers.
• It’s dangerous to drive on icy streets. The streets are icy now, so it would be dangerous to drive on the streets.
• All cats have a keen sense of smell. Fluffy is a cat, so Fluffy has a keen sense of smell.
• Cacti are plants, and all plants perform photosynthesis. Therefore, cacti perform photosynthesis.
• Red meat has iron in it, and beef is red meat. Therefore, beef has iron in it.
• Acute angles are less than 90 degrees. This angle is 40 degrees, so it must be an acute angle.
• All noble gases are stable. Helium is a noble gas, so helium is stable.
• Elephants have cells in their bodies, and all cells have DNA. Therefore, elephants have DNA.
• All horses have manes. The Arabian is a horse; therefore, Arabians have manes.

Invalid Deductive Reasoning

When it comes to deductive reasoning, you can overgeneralize. In these cases, even with two solid and true premises, deductive reasoning goes wrong. Here are a few examples of just that:

• All swans are white. Jane is white. Therefore, Jane is a swan.
• All farmers like burgers. Jethro likes chicken wings. Therefore, Jethro is not a farmer.
• All actors are handsome. Tom Cruise is handsome. Therefore, Tom Cruise is an actor.

In each of these examples, the premises may very well be true, but the conclusions make invalid assumptions. In these examples, a + b does not necessarily equal c. Rather, “c” is an overgeneralization.

Let’s take the Tom Cruise example. Just because Tom Cruise is handsome, does that mean he must be an actor? Who’s to say all electricians or writers aren’t pretty, too?

Deductive Reasoning vs. Inductive Reasoning

Inductive reasoning is akin to deductive reasoning. The main difference is that, with inductive reasoning, the premises provide some evidence for the validity of the conclusion, but not all.

With deductive reasoning, the conclusion is necessarily true if the premises are true. With inductive reasoning, the conclusion might be true, and it has some support, but it may nonetheless be false. However, your educated guess can become a hypothesis you could consider fleshing out through research and an abundance of outside sources.

Examples of Inductive Reasoning

Let’s take a look at a few examples of inductive reasoning. After we examine the inductive reasoning, we’ll flip it and see what it looks like in the form of deductive reasoning.

• Inductive Reasoning: The first lipstick I pulled from my bag is red. The second lipstick I pulled from my bag is red. Therefore, all the lipsticks in my bag are red.
  Deductive Reasoning: The first lipstick I pulled from my bag is red. All lipsticks in my bag are red. Therefore, the second lipstick I pull from my bag will be red, too.
• Inductive Reasoning: My mother is Irish. She has blond hair. Therefore, everyone from Ireland has blond hair.
  Deductive Reasoning: My mother is Irish. Everyone from Ireland has blond hair. Therefore, my mother has blond hair.
• Inductive Reasoning: Most of our snowstorms come from the north. It’s starting to snow. This snowstorm must be coming from the north.
  Deductive Reasoning: All of our snowstorms come from the north. It’s starting to snow. Therefore, the storm is coming from the north.
• Inductive Reasoning: Maximilian is a shelter dog. He is happy. All shelter dogs are happy.
  Deductive Reasoning: Maximillian is a shelter dog. All shelter dogs are happy. Therefore, he is happy.

Notice how each example of deductive reasoning is more sound (assuming the first two premises are true)? In each instance, the inductive reasoning may be true. But, they’re lacking enough evidence to be universally true. Further samplings would be required.

Other Types of Reasoning

Deductive and inductive reasoning aren’t the only type of reasoning. You might also come across abductive reasoning, backward induction, and critical thinking. Let’s look at what these types of reasoning are:

• Abductive reasoning is when you take a set of observations and use a theory to explain them. This is very similar to how doctors work on patients by taking symptoms to make a diagnosis.
• Backward induction looks at the end result and considers the different decisions that lead to that conclusion. It’s used by artificial intelligence to win games.
• Critical thinking uses analysis and evidence to make an informed decision. It’s used daily to make decisions and to analyze decisions in science, literature, etc.

Don’t Leave Room for Assumptions

If you proceed with facts and evidence, your deductive or inductive reasoning can quickly turn into an assumption. And that’s what we typically try to avoid in life. A hypothesis, however, is a nice place to start. This is an idea that can be molded into factuality and follow the lines of deductive reasoning.

That might be a road worth considering if you’re ever tasked with writing an argumentative essay. Of course, the goal is not to get into an argument but, rather, take a position and present evidence in support of your claim.