I am currently confused, below I have a statement and my expression of that statement in first order logic.


W(person, song)

R(person, song, album)

Statement: Every song that Gershwin wrote has been recorded on some album.

Answer: ∀y W(G, y) ⇒ ∃a R(G, y, a)

However, my textbook answer

Answer: ∀y W(G, y) ⇒ ∃a,p R(p, y, a)

Instead of using Gershwin in the argument of R, why did it instead say there exists a person p? Does it really make a difference or am I still unclear about how expressing statements in first order logic work?

  • 1
    $\begingroup$ Because $p$ is the person who recorded song $y$. So, if $y$ stands for "It's Wonderful," $p$ could be Barbara Hendricks. $\endgroup$ – Fabio Somenzi Nov 3 '18 at 5:51
  • $\begingroup$ Indeed, after David pointed it out below I facepalmed myself. $\endgroup$ – Belphegor Nov 3 '18 at 5:53
  • $\begingroup$ Apparently in R(person, song, album) the variable "person" stands for the recording artist, not for the composer. The textbook answer comes down to: for every song $y$ , if it is composed by Gershwin, you can be certain that it is recorded on an album $a$ by an artist $p$. $\endgroup$ – M. Wind Nov 3 '18 at 5:53

Because there is no guarantee that Gershwin recorded the song. Someone else might have.

  • $\begingroup$ Oh damn! You're right. $\endgroup$ – Belphegor Nov 3 '18 at 5:50
  • $\begingroup$ I was under the assumption that Gershwin recorded the song too, but clearly that's not the case after you pointed out the obvious. I should be more careful on my part after this. $\endgroup$ – Belphegor Nov 3 '18 at 5:52

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