In Allison Hills' paper "Moral Testimony" in Philosophy Compass, she refers to: i) p and ii) p'.

This is in the context of discussion one's ability to reason about p and q such that one can: "draw the conclusion that p'...from the information that q' (where p' and q' are similar but not identical to p and q)".

My question is, what logical effect does the apostrophe have when modifying p to p'?

Thank you for your help!

  • 3
    It is not a logical symbol. The author is just using p' to indicate a proposition similar to p, and q' similar to q. In context, the claim is that a person does not understand why p follows from q unless they also grasp why p' follows from q', where the apostrophe indicates some relevant corresponding similarity in both propositions.
    – Bumble
    Feb 22 at 18:12

1 Answer 1


This is a "prime" not an apostrophe, and is read as "P prime":

In general it denotes a variation, but the exact meaning has to be defined in context because there's no universal meaning to it. You generally use it when you have two variables, and one is related to the other in a way that you want to discuss.

In this context, it is defined as meaning "similar to." So p' is similar to p but not identical with it. It is further understood that q' stands in the same relationship to q. This would be a typical use of the prime.

  • Thanks Chris, this is very helpful!
    – MER
    Feb 23 at 4:42

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