The Difference Between Soundness And Validity

Have you ever heard people distinguish between calling an argument sound and declaring it valid?  If so, did you understand the difference between the two labels?
If not, I hope this helps illuminate the issue.  I haven't really commented on this topic yet but decided that it is important to establish.

An argument is valid if the conclusion follows from the premises; an argument is sound if the premises and resulting conclusion are actually true (and if the argument is valid as well).  To see a syllogism that is valid but unsound, see below:


1. If Abigail loves Samantha, she cannot hate Samantha.
2. Abigail loves Samantha.
3. Therefore Abigail cannot hate Samantha.


This is an example of a syllogism that is logically valid but unsound.  While it is true that if premise one and two are correct that the conclusion above follows inescapably, it is not true that it is impossible to love and hate someone at the same time.  To borrow a movie example, in The Fellowship Of The Ring Gandalf says to Frodo that Gollum hates and loves the ring as he hates and loves himself.  Just because you love something does not mean you do not loathe it simultaneously.  There is no logical contradiction between loving and hating something or someone despite such a contrary notion existing in present American secular and church culture.  Thus, this argument possesses validity but not soundness.

Now, inspect the following:


1. If Jesus was divine, then Christianity is true.
2. Jesus may have been divine.
3. Therefore Christianity may be true.


This is an example of a syllogism that is both valid and sound; the conclusion follows from both premises and the premises and conclusion are entirely correct.  The argument meets both sets of criterion.

I make this distinction between soundness and validity for the sake of future reference and because it is necessary to clarify these things when discussing logic.  Now, I have heard people classify these two terms with the inverse definitions, and if I have the two backwards I am willing to edit this post and switch them.  This type of distinction serves to better utilize language and terms in discussions and helps with clarification of specific points that need precise handling and description.

Feel free to comment and correct me if I mistakenly swapped the two definitions!