Deductive reasoning is at the core of troubleshooting, which is essentially the art of investigation. In fact, the most famous deductive reasoner might be ace investigator Sherlock Holmes.
Deductive reasoning is the logical thinking process, which goes from general to specific. It is based on premises, and if the premises are true, then the reasoning will be valid. The reasoning statement starts with the premise and often uses an if/then format. For example:
If all squares are rectangles and all rectangles have four sides, then squares must have four sides.
If snakes are reptiles and reptiles are cold-blooded, then snakes are cold-blooded.
If 5 + a = 9 then a must be 4.
It's also possible to eliminate facts that are irrelevant to the deduction:
If both long and short snakes are reptiles, and all reptiles are cold-blooded, then snakes are cold-blooded.
All snakes are cold-blooded, their size is irrelevant to the logical conclusion.
Another example: A customer brings in an iPhone and says, "My iPhone won't power on. I often skip breakfast. I haven't charged it in days. I use Siri all the time."
Of the information provided, the fact that the customer skips breakfast seems irrelevant to the power issue.
The ability to narrow down the information set and draw logical conclusions is the DNA of the troubleshooting and diagnostic process. To diagnose an issue you need to understand the issue's circumstances and history so you can work with relevant facts, and then discard the distractors or irrelevancies. From the relevant facts you can deduce likely causes.
As Sherlock Holmes said, "Eliminate all other factors, and the one which remains must be the truth.