Aristotle described a system of logic that was so iron-clad, it was 2000 years before philosophers and mathematicians started seriously adding to it. It is famous for defining the syllogism which is a form of deductive logic, where certain conclusions can be mathematically proved to be true as long as the starting premises are true. The prototypical example of a syllogism is:
P1: All men are mortal.
P2: Socrates is a man.
C: Therefore, Socrates is mortal.
As long as the two premises P1 and P2 are true, C is guaranteed to be true.
However, unlike deductive reasoning, everyday situations usually need to come up with conclusions with less than absolute provability. Jurors need to come up with a verdict even though they have not been given iron-clad arguments. One of the several types of reasoning that is not deductive is called defeasible logic.
Many everyday arguments rely on assumptions that are true "all things being equal". For example, if told that birds were being transported in boxes with no lids, you might conclude that there was a large risk that the birds would fly away, and therefore lids were needed. Given what you know, this is a reasonable conclusion. However, those arguments are defeasible, which means that there are other additional statements that could change the conclusion if they were added to the mix. If we add the statement "the birds being transported are penguins", the conclusion that they might fly away would be defeated since penguins can't fly. On the other hand, some arguments are indefeasible. For example, "John is a bachelor therefore John is unmarried" would not be false, no matter what additional statements are made because being unmarried is the very definition of being a bachelor.
It is helpful to know this little distinction between defeasible and indefeasible conclusions, as is shown in the following little case study.
Case Study: Form Input Validation