According to Aristotle, there were three basic commonplaces that speakers could appeal to in order to make a case. These commonplaces established a particular path for an argument, and so they were considered part of logos, to some degree. But these were not, for Aristotle and Quintillian, the arguments of "experts" so much as "common sense." In Latin, the phrase we translate as "commonplaces" is locis communis. The word communis is etymologically similar to English words common and community. That connection is significant because ancient writers and thinkers believed that the commonplaces were ways for anyone to make arguments. In fact, earlier, when we looked at logos, we saw that often the most "believable" arguments are the commonsensical ones. In America today, we still borrow from such commonplaces as the ones Aristotle offered us more than two centuries ago.
In his Rhetoric, Aristotle discusses "educated guesses" that people can make about past or future actions based on their present knowledge about the way the world works. It is important to realize that when Aristotle discusses "facts" he is not thinking of "fact" as contemporary Americans would (see below). Actually, he's talking about conjecture or assumptions that can be made. For example, based on our present knowledge of how rivers flow in a particular direction, we can make assumptions that in the past, rivers flowed in this same direction. Although we cannot know such a thing because we have not been there and thus have no immediate proof, we can make an assumption, and such an assumption would be very convincing to readers/listeners because of the "common knowledge" of rivers. Likewise, we could project that rivers, in the future, will also flow in a particular direction to the ocean.
Greater/Lesser involves the degree of something, and therefore is related to a system of values. The argument could be made from the lesser (smaller group or number) to the greater (larger group or number), or vice versa. Consider the following argument from degree: "We have all seen that Timothy's behavior has significantly improved since his parents gave their permission to the school to spank him. Likewise, the same improvement in behavior has been noted in Victoria, Quinton, and Christopher after their parents gave similar permission to the school. Clearly, spankings would improve the behavior problems that all elementary school students demonstrate." Here, I have made an argument that because the lesser (Timothy, Victoria, Quinton, and Christopher) has experienced improved behavior, the greater (the rest of the students in elementary school) would also experience improved behavior. Such cases are often made, but they rarely take into account the specificity of each child's situation.
Arguing probability requires a rhetor to make cases about what
is possible or impossible. For example, local government decides to adopt
school policy in which there would be a teacher to student ratio of 1:5 in
each classroom. Although we might desperately want such a thing to happen,
those who are opposed to it could easily argue that it simply isn't possible
for any number of reasons: the budget won't pay for that many new teachers;
there aren't enough qualified teachers in the workforce to fill that many
positions; there's not enough money to pay for the additional space that that
many new teachers would take up in schools. All of these arguments are based
on what is possible or impossible.
One of the things we've seen earlier is that modern notions of "facts" are quite different from classical notions, especially in rhetoric. Many classical rhetoricians argued that rhetoric was not the realm for irrefutable proof; instead, rhetoric was about convincing people of something when there seems to be no absolute right answer. However, in the twentieth century, we're much more interested in facts that are irrefutable. We constantly ask for statistics and numbers to validate something. Early in the century, there were court cases that tried to determine if African Americans were as fully human as European Americans. Often, these cases asked for scientific studies that could supply numerical data one way or the other. Such debates continue in our public schools where we constantly ask for standardized test scores to determine both the intelligence of our children and the quantity of knowledge they've consumed in a given time period. The commonplace assumption is that "number don't lie." Of course, number can be misleading, particularly out of context. Remember the example above about the 90% of students who claimed that Watternson's food was great (Hasty Generalization).
Another modern commonplace that developed through writers and thinkers like Oscar Wilde was that art should exist just because it's art, not because of what people think of it. More recently, we've even had the concept of "modern art," which argued that the painter's or sculptor's intentions are irrelevant; his/her "meaning" doesn't matter. What matters, instead, is what the person who sees the piece believes it to mean. Such a position, of course, is a bone of contention still. There are those who believe all art should have a particular meaning, and there are those who say no. This same argument exists in English departments in the form of Reader Response criticism. There are adherents to Reader Response who claim that any text means only what the reader gets out of it; others believe that a text has a meaning and we must find it. In either case, the modern commonplace has come to be that "it's just my opinion," and many of us have ceased to find any way of combating such a stance.
Of course, there are many commonplaces that contemporary folks uses on a regular basis. These commonplaces rarely need explanation because, like those in Aristotle's day, they represent the common sense of the masses and thus make arguments without having to really do much work. We might call such commonplaces ideologies, as Sharon Crowley and Debra Hawhee have done in their book Ancient Rhetorics for Contemporary Students. They give the following examples of groups who sport different (and often conflicting) ideologies: Christians, Jews, Atheists, Marxists, Feminists, socialists, capitalists, Southerners, Midwesterners. The lists could go on, but what the contemporary student of rhetoric would recognize is that each of these groups (and countless others) often make arguments based on common assumptions.
For example, people in both the Ku Klux Klan (KKK)and the National Association for the Advancement of Colored People (NAACP) make arguments about the "right" of individuals in America to "free speech." However, these two groups are radically opposed in other areas of their belief system. Such an example points out that our commonplaces are not universal — not every country believes in the right to free speech — but instead contingent upon other factors. However, within certain places, like America, anyone can call on the commonplace of the right to free speech to make an argument that might be influential to other listeners because the argument is based on assumptions that people believe in.