We will understand the difference between these concepts through some examples. Consider the argument below
Every human being is mortal.
Socrates is a human being.
So Socrates is mortal. p>
It does not take much analysis effort to realize that this is a good argument. In fact, we can conclude with all certainty that Socrates is mortal if the premises of the argument are true.
Now compare the argument above with the next one:
All mammals are animals.
Whales are mammals.
So whales are animals.
If we analyze the content of the above arguments, we will see that they are different. One talks about human beings and their mortality, another about mammal animals and whales in particular.
However, from another point of view, there are similarities between them. This similarity is in the logical structure of the argument. One way to highlight this similarity is to replace the argument’s terms with variables.
The result is as follows:
All A (human beings) is B (mortal).
C (Socrates) is A (human).
Therefore, C (Socrates) is B (mortal).
All A (mammals) is B (animals).
C (whales) is A (mammal).
C (whale) is B (animal).
Therefore, the structure or logical form of the above arguments is
Every A is B.
Every C is A.
So C is B.
Logic is interested in the form of the arguments, because one is an argument is valid or invalid depending not on the content of the statements that compose it, but on its structure.
The above arguments are clearly valid. In addition, any argument that has the same form will be valid, regardless of its content.
Walton, Douglas. Informal Logic: A Pragmatic Approach. Cambridge: Cambridge University Press, 2008.