Aristotle was perhaps the original fan of logic as a discipline. Previous philosophers had recognised logic an argument, but Aristotle was, by comparison, obsessed. His main interest was in inductive logic, that is, given a series of events (such as the sun rising every day), then the next event can be predicted (so the sun will rise tomorrow). This may be the type of logic that we use most, but it is of limited use. In the example, the sun will eventually not rise one day.
Arthur Conan Doyle’s Sherlock Holmes would solve crimes by the use of what he claimed was deductive logic. Holmes’s method of detection was to decide between theories of how a crime was committed by eliminating the impossible theories until only one theory (no matter how improbable) was left. This is, however, not deductive logic, but abductive logic. One issue with this is type of logic is simply, what if there is another theory that fits the facts but which has not been considered?
Deductive logic, in its simplest form, starts with premises, that is, alleged facts and is used to argue for a third fact (providing rules are followed) or conclusion. I say alleged as the premise may be false. Socrates was a man and all men are mortal are facts and the conclusion that Socrates was mortal is a true conclusion. If instead we claim that all men speak with Irish accents, we get the conclusion that Socrates spoke with an Irish accent, which is wrong.
To come to the conclusion, that is, show, that Socrates was mortal, we had to know that Socrates was a man. This is an example of a necessary reason. It was needed to show that he was mortal.
In this case, it is also a sufficient reason to show Socrates’s mortality.
Let’s turn to something a little more difficult. Why are football, chess, hide-and-seek and Suduko all games, when all are so different? Did people just attach the word game to all is there one fact about each of them that marks them out as all being games (and other activities being not games). All have rules, but so does logic and this is not a game. Is there one feature? Why has no one found that one feature?