Enthymeme

The term enthymeme refers to any informal argument that includes unstated assumptions. The term was theorized by Aristotle. As this link points out, enthymemes deal with probabilities or likelihoods for events. Unlike a syllogism that confirms an inevitable conclusion, an enthymeme can (by definition) illustrate either an inevitable conclusion or a conclusion that is most likely.

Therefore, an enthymeme can be a valid form of deduction or a logical fallacy. As a form of deduction, an enthymeme can logically illustrate cause/effect relationships; as a logical fallacy, it is a categorical syllogism that omits a premise and/or conclusion. The missing proposition is considered to be implied.

More specifically, enthymemes are very commonly used in everyday argumentation because the mind usually supplies the missing premise unconsciously; however, enthymemes can be used to obscure a bad argument. Therefore, when constructing an enthymeme, one must make sure that the missing premises and/or conclusions are valid. Otherwise, the person composing the enthymeme could commit the syllogistic fallacy, which is a formal fallacy.

Alternate names

 * jumping to conclusions

Examples
A non-fallacious example would include:

The puppy will chew up your shoes because you left the closet door open in your bedroom.

This statement assumes that puppies (generally speaking) chew up objects they find in the house. In this case, it is reasonable to say that a puppy will (most likely) chew up objects around the house. Given the ubiquity of this knowledge, the enthymeme is a sufficient means for communicating the argument. In other words, the person making this argument can omit the general/major premise that puppies chew up objects.

This can be restated formally as:

René Descartes' "Cogito ergo sum" is also an enthymeme which omits its general/major premise and is a sufficient means for communicating the argument given the ubiquity of the knowledge of this premise (although it is much debated whether the argument itself is fallacious):

Santa
However, enthymemes can also be fallacious. A fallacious example might be something like this:


 * Santa will give you presents because you've been a good child.

This statement assumes that Santa gives presents to good children. This can be restated formally as:

However, while the elaborated syllogism does follow the form, this enthymeme jumps to conclusions in one way: it assumes that Santa Claus exists. Unlike a puppy, the existence of which we can prove, no one has proven that Santa Claus exists.

In essence, these two examples illustrate how enthymemes can either be logical (e.g. the puppy enthymeme) or fallacious (e.g., the Santa enthymeme).

War
Let's look at another example that illustrates how enthymemes can also be both logical and fallacious.
 * War causes death.
 * Therefore, war is evil.

On the one hand, it is a fact that war causes death. In this regard, the enthymeme has a logical basis. We could even concede that war is generally evil. However, there is a missing premise, "All that which causes death is evil," that is required to make the logical connection between the first premise and the conclusion. It is important to recognize enthymemes and analyze the implied premise to ensure no false premises are being "slipped under the rug." Another way to put this: War could be "evil" for a lot of reasons: it divides families, stretches natural resources, costs taxpayers money, etc. In other words, while each statement expresses general truth as it were, they are not sufficiently connected to make this enthymeme necessarily logical.

Apologetics
Fundamentalists frequently try and fail to convert people by saying:
 * If you don’t believe the Bible, you're going to Hell.

Of course, they ignore the necessary unstated premises of:
 * God exists.
 * The Bible is the sole representative record of His word.
 * God will send those who don't believe in His word to Hell.

But whenever this is pointed out, they go "La la la, I can’t hear you!"

Avoiding the fallacy
Someone can accuse you of jumping to conclusions if you assume something is true without stating it outright, or someone may accuse you of jumping to conclusions merely because he wants to do it, regardless of any commitment to logic. If you wish to convince an audience, you may consider spelling out any assumptions that might not be shared by your audience. But note that you might be asked to defend those assumptions, so it would be wise to prepare what you will say in response and avoid committing a fallacy during the hasty construction of a response. But realize that once you give some proof for an assumption, you have essentially abandoned the assumption, admitting that it is not self-evident and thus not assumable. Logic can assure that your conclusions are valid based on assumptions (givens); but logic has no power whatever to create the assumptions (givens, self-evident truths, axioms). As such, those who create enthymemes must make sure the missing premises are indeed logical. Otherwise, the assumptions will be based on fear, prejudice, anecdotal evidence, and other untested and/or illogical assumptions.