Editing Modus tollens (section)

== Justification via truth table ==
The validity of ''modus tollens'' can be clearly demonstrated through a [[truth table]].

{| class="wikitable" style="margin: 0 auto; text-align:center; width:45%"
|-
! style="width:15%" | p
! style="width:15%" | q
! style="width:15%" | p → q
|-
| T || T || T
|-
| T || F || F
|-
| F || T || T
|-
| F || F || T
|}

In instances of ''modus tollens'' we assume as premises that p → q is true and q is false. There is only one line of the truth table—the fourth line—which satisfies these two conditions. In this line, p is false. Therefore, in every instance in which p → q is true and q is false, p must also be false.