In the truth-table and Venn diagram method, we can use letters to represent parts of the arguments because the validity of the arguments doesn't really depend on what the letters represent. This makes the arguments formally valid. Some arguments aren't formally valid, because they depend crucially on the information that they contain, not on the form or structure of the argument. One kind of such arguments is the inductive argument. Most of what we know about the world comes from induction.
Induction is the formal name for learning from experience.
For example, any statistical information is inductive in nature. Drug research tries to generalize about all people by trying a drug out on only a few. School examinations generalize about your understanding of all facets of a class by testing you on a few. Quality control testing generalizes about the fitness of all products by testing a limited number. Computer programmers try to correlate a particular part of a program with unintended behavior to show that this part of the program causes the incorrect behavior.