It assumes that $P$ and $Q$ are correct mathematical expressions. The sentence $P\rightarrow Q$ has associated sentences
$Q\rightarrow P$ $\hspace{15mm}$reciprocal of $\hspace{20mm}P\rightarrow Q$;
$\neg P\rightarrow \neg Q$ $\hspace{8mm}$ inverse of $\hspace {25mm}P\rightarrow Q$;
$\neg Q\rightarrow \neg P$ $\hspace{8mm}$ counter-reciprocal of $\hspace {7mm} P\rightarrow Q$.
Pairwise have the same truth table. What are these partners?
[Hint. Use truth tables]
