Conjunction and Implication

Modus Ponendo Tollens (abbreviated $\textrm{MPT}$)
The following can also be seen to hold:

which follow immediately from
 * $\left({p \land \neg q}\right) \iff \left({\neg \left({p \implies q}\right)}\right)$

and the rule of simplification.

Proofs
By the tableau method of natural deduction:

Proofs using the LEM
The remaining proofs depend (directly or indirectly) on the Law of the Excluded Middle.

Comment
Note that the Modus Ponendo Tollens:
 * $\neg \left({p \land q}\right) \dashv \vdash p \implies \neg q$

can be proved in both directions without resorting to the LEM.

All the others:


 * $p \land q \vdash \neg \left({p \implies \neg q}\right)$
 * $p \implies q \vdash \neg \left({p \land \neg q}\right)$
 * $p \land \neg q \vdash \neg \left({p \implies q}\right)$

are not reversible in intuitionist logic.

Proof by Truth Table
We apply the Method of Truth Tables.

As can be seen by inspection, the truth values under the main connectives match for all models.

$\begin{array}{|cccc||cccc|} \hline p & \land & \neg & q & \neg & (p & \implies & q) \\ \hline F & F & T & F & F & F & T & F \\ F & F & F & T & F & F & T & T \\ T & T & T & F & T & T & F & F \\ T & F & F & T & F & T & T & T \\ \hline \end{array}$