Deductive Reasoning

Deductive reasoning is, of course, a way of reasoning. We use deduction to reach conclusions that we can be sure about based on the premises. But how is deductive reasoning different from other logical processes like inductive reasoning or abductive reasoning? Perhaps it’s easiest to use a (tasty) example to contrast deduction with other reasoning methods. Imagine you’re in the kitchen, looking at the cookie jar…

In deductive reasoning, also known as top-down logic, you start with a general rule and apply it to a specific case to reach a logically certain conclusion.¹ Maybe you only ever bake chocolate chip cookies, so you know for certain that, as a general rule, all cookies in your jar are of the chocolate chip variety. In this specific case, you’ve blindly grabbed a cookie from the jar. As long as your general rule is true, you can conclude with certainty that the cookie you took from the jar will be chocolate chip. Deduction is like following a recipe exactly and knowing the cookies will turn out a certain way. If you always use a chocolate chip recipe, you can expect that your cookies won’t magically end up as oatmeal raisin. 

In inductive reasoning (bottom-up logic), on the other hand, you observe specific cases and create a general rule, though it’s not always 100% certain. Perhaps you first observe that the first three cookies you took from the jar were chocolate chip. The pattern is that every cookie so far is chocolate chip. You might conclude that all the cookies in the jar are likely to be chocolate chip, even though this isn’t necessarily true. Induction is like tasting a few cookies and guessing that the whole batch is the same, even though there might be a surprise oatmeal raisin hiding!

Finally, in abductive reasoning (best guess logic), you take an incomplete set of observations and come up with the most likely explanation. Maybe you observe that there are cookie crumbs on the table, and your little brother has chocolate on his face. The best explanation is that your brother ate the cookies. Abduction is like being a cookie detective and making the best guess about what happened, even though you don’t have all the facts.

So, in summary:¹

• Deduction = Fixed Rules → Certain Conclusion 
• Induction = Observed Patterns → Probable Conclusion
• Abduction = Available Clues → Best Guess

Deductive reasoning is valuable because it provides certainty; unlike inductive or abductive reasoning, as long as the premises (like your personal rule of only ever baking chocolate chip cookies) are true, and as long as the structure of your reasoning is sound, then deduction will provide logical certainty.¹