Deductive reasoning is the process by which you come to a conclusion by starting with the
cause and ending with the
effect.
Sherlock Holmes was quite a fan of it. It is the
formulation of a
theory, the gathering of
evidence (via
observation), and the
confirmation or
dismissal of the
theory.
If a
co-worker were to walk by your desk just after lunch break and say "Too bad you missed lunch, today", without having any way of knowing this, you would likely be
puzzled.
Their deductive line of thought, however, might be as such:
Theory:
John Doe didn't go to lunch today.
Evidence:
Bank slip on
John Doe's desk is dated today.
John Doe was at his desk until lunch.
It is
Friday, and the banks would be
jammed.
John would not have had time to eat at a
restaurant.
There is no litter around the desk to indicate take-away food.
John is not currently eating.
Confirmation:
John Doe did not go to
lunch.
The leap from submittal of evidence to confirmation of theory is called a "
deductive leap". This manner of decoding events is most often used in science and law.