An enthymemes is argument with an unstated premise. It’s a common form of reasoning in everyday discourse where certain parts of the argument are left implicit.
For example, consider the following statement:
“Socrates is mortal because he’s human.”
If we translate that into formal logic, we get
which is clearly invalid.
The reason is that we didn’t put in the unstated promise that “all humans are mortals.” If we put it into the argument, we get a valid argument: