Whenever, therefore, people are deceived and form opinions wide of the truth, it is clear that the error has slid into their minds through the medium of certain resemblances to that truth.—Socrates
Here are some examples of simple is-ought statements. Tony is tall; therefore he ought be a basketball player. Everybody likes fun; so you must buy some beer. Crime is rife; we ought to make our punishments more severe. Carl is crazy; he should be locked up for a year. We have the best holy book; therefore we must believe what it says, and we must agree that Albert Einstein was wrong about all that science-stuff.
The first example begins with a single claim or assertion, namely, "Tony is tall." The claim is descriptive; that is, it purports to be a fact. The descriptive claim is followed by the conjunctive adverb therefore. After therefore comes a prescriptive claim. This claim offers an opinion; it says what should be done or what ought to be believed.
An is-ought statement may have several descriptive claims. None of those claims has to use the word is. The descriptive claim can use any form of the verb to be or any form of other verbs. Often, the descriptive claim uses a form of the verb to have. For instance, the statement about Tony could be stated as: Tony has the height of a basketball player; therefore he should be a basketball player.
Often, one finds a bamboozler using therefore, so, thus, or then to link a descriptive claim to a prescriptive claim. Sometimes, instead of an adverb, a hoodwinker will use a statement such as It follows or We must conclude. As the examples illustrate, one does not have to use any link between the is-claim and the ought-claim.
Positioning a prescriptive claim after a descriptive claim is a rhetorical trick. The nearness of the claims implies that one is relevant to the other and that the prescriptive claim is a logical consequence of the descriptive claim. The idea is that, if one accepts the truth of the descriptive claim or claims, one must accept the reality of the prescriptive claim or claims.
In fact, the ought-claim is not a logical consequence of the is-claim. There is no way to know if an ought-claim is a good idea from knowing only what the is-claim is because an is-claim always allows more than one ought-claim. It is impossible to go logically from "is" to just one "ought." (This observation is known as Hume's Guillotine because Scottish philosopher David Hume mentioned it first.)
Consider this is-ought statement: the grass is tall; so we should cut it. To decide whether cutting the grass is a good idea, we need to remove the descriptive claim and the adverb so. Then we have to ponder our options; we have to consider the advantages and disadvantages of cutting the grass. We might conclude that we should cut the grass; but we could also conclude that we should let the sheep eat it or that we should wait until the hailstorm stops or that we should repair the lawnmower or that we should pray for the grass-pixies to make the grass shorter. Without more information, we cannot make a logical decision about which ought-claim is best.
Sometimes, an is-ought statement is disguised. For example, its claims may be formulated as questions rather than as assertions. Consider the following questions. How can anyone deny that disease exists? How can anyone deny then that there are demons? To understand what the hoaxer is saying by these questions, we might reformulate his words thusly: disease exists; therefore we must believe in demons. Neither of the questions nor the reformulation explains why we should believe in demons. The ought-claim does not tell us why we ought to believe what the hoaxer says. Is-ought statements do not address the question: why!
Above, I have called the person who uses is-ought statements a bamboozler, a hoodwinker, and a hoaxer. I did so because the purpose of the is-ought statement is often to deceive. Deception is not, however, the intent of everyone who uses is-ought statements. Some writers or speakers use the statements for the sake of convenience. For instance, such statements can allow a writer or a speaker harmlessly to bring a topic or a discourse to an end. I will omit an example because this essay is long enough. It ought to end here.