Greek philosopher Aristotle is often credited with being one of the first to document reasoning that mirrors what we call deductive reasoning today. Plato and Socrates are also in the mix of deductive reasoning creators, and there's early Egyptian and Babylonian evidence providing support for even earlier uses. The truth is, whether it was documented or not, humans have used deductive reasoning for many millennia.
Here are two simple examples, perhaps from the time of cavemen:
“When water falls from the sky, and the pond fills with water, I have water to drink. Water is now falling from the sky, and the pond is filling with water, so I will have water to drink.”
“When I touch a fire, it hurts. There is a fire over there. If I touch it, it will hurt.”
Some modern examples:
“When my global positioning system (GPS) says to drive a certain route and I drive a different route, my GPS says, ‘Recalculating.’ Oops, I just missed my turn, and I'm going to drive a different route; therefore, my GPS will say, ‘Recalculating.’”
“Every new employee must go to new employee orientation. We just hired a new employee, so he must go to new employee orientation.”
“I'm holding a cup of only red marbles. If this statement is true, any marble I take out of the cup will be red.”
The initial statement in deduction defines a general truth. Based on that truth, we can determine a specific instance is true as well. The classic example: all people (persons) are ...