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2.3 Co-premises

Multiple premises working together as part of a single reason or objection are known as co-premises. A co-premise helps connect ("co-nnect") the other premise, or premises, to the contention. 

Two premises are part of the same reason or objection when they are working together. Roughly, two premises work together when each one needs the other so that they can, as a group, provide evidence for (or against) the contention.  They don't stand alone; rather, they work hand in hand to show that the contention is true (or false).

Example:

You can see how when both of these premises are true, you have a reason to believe that the Apollo astronauts landed on the Moon.  However if either of them is false, the other one is not enough on its own.  Thus, the premises are working together.

Another intuitive way to understand this notion of co-premises working together is to think of a co-premise as what connects the other premise(s) to the contention.  It makes the connection or bridge between claims which otherwise might seem unrelated or only partially related. 

Don't worry too much if you're not 100% clear yet on this notion of "working together."  We will clarify it a lot in the following pages and tutorials.  However it is worth mentioning right now that this is one of the most difficult of all the concepts we will deal with.  Getting it right is one of the biggest challenges in argument mapping!

Technical point: The notion of co-premise is relative. Nothing can be a co-premise on its own, just as you can't be a husband or a sister on your own.  Husbands are only husbands in relation to wives, and sisters are only sisters in relation to their siblings. Likewise, premises are co-premises in relation to other premises, as long as are part of the same reason or objection.  Two premises are not co-premises in relation to each other if they belong to different reasons or objections.

Alternative terminology

Logicians use various terms to refer to co-premises.  Some others you might come across are:

• linked premises
• helping premises (Reason!Able software)

Co-premises: Two premises within a single reason or objection are co-premises in relation to each other.