Therefore, God exists. Assume that the premises of all of them are true. It appears that all I mean by this is that either Clinton is not a great president, or I'm a monkey's uncle. I rely on the audience to realize that I am not a monkey's uncle. From thence they are supposed to infer that Clinton is not a great president.
All men are mortal.
Therefore, Socrates is mortal. Clearly this is a valid argument. All of these sentences have to be represented as atomic sentences, because they don't contain any connectives. Now, it's pretty easy to 'prove' that the argument, S A M is not valid, using the method of truth tables. We look for a line on which both premises are true and the conclusion is false.