A co-premise is a premise

Premise

Premise can refer to:* Premise, a claim that is a reason for, or an objection against, some other claim as part of an argument...

 in reasoning and informal logic

Informal logic

Informal logic, intuitively, refers to the principles of logic and logical thought outside of a formal setting. However, perhaps because of the informal in the title, the precise definition of informal logic is matters of some dispute. Ralph H. Johnson and J...

 which is not the main supporting reason for a contention or a lemma

Lemma (logic)

In informal logic and argument mapping, a lemma is simultaneously a contention for premises below it and a premise for a contention above it....

, but is logically necessary to ensure the validity

Validity

In logic, argument is valid if and only if its conclusion is entailed by its premises, a formula is valid if and only if it is true under every interpretation, and an argument form is valid if and only if every argument of that logical form is valid....

 of an argument

Argument

In philosophy and logic, an argument is an attempt to persuade someone of something, or give evidence or reasons for accepting a particular conclusion.Argument may also refer to:-Mathematics and computer science:...

. One premise by itself, or a group of co-premises can form a reason.

Logical structure

Every significant term or phrase appearing in a premise of a simple argument, should also appear in the contention/conclusion or in a co-premise. But this by itself does not guarantee a valid argument, see the fallacy of the undistributed middle

Fallacy of the undistributed middle

The fallacy of the undistributed middle is a logical fallacy, and more specifically a formal fallacy, that is committed when the middle term in a categorical syllogism is not distributed in the major premise...

 for an example of this.

Sometimes a co-premise will not be explicitly stated. This type of argument is known as an 'enthymematic' argument, and the co-premise may be referred to as a 'hidden' or an 'unstated' co-premise and will often be subject to an inference objection

Inference objection

In informal logic, an inference objection is an objection to an argument based not on any of its stated premises, but rather on the relationship between premise and contention. For a given simple argument, if the assumption is made that its premises are correct, fault may be found in the...

. In this argument map of a simple argument the two reasons for the main contention are co-premises and not separate reasons for believing the contention to be true. They are both necessary to ensure that the argument as a whole retains logical validity.


In this example, "What the Bible says is true" is a hidden co-premise.