7sage, the definition of "subject" being "the thing that the sentence is about" is very misleading. The subject of a sentence is the thing doing the action.

I'm having trouble with the term "assumption" as it is applied to this particular argument. "Tigers are mammals" is not an assumption, it is implicit in the argument because it is fact; there is no ambiguity surrounding its veracity. Why would it matter whether this fact is included in the argument? In other words, the statement is (by default) not subject to this spectrum of "reasonability." Please help me understand.