Act 3 begins with Martha Corey telling the court that she is not a witch, but Judge Hathorne asks her "How do you know, then, that you are not a witch?" What is strange about this logic?

This is unusual because one is more likely to be not a witch than be one as no one knows what makes a witch for certain.

Explanation:

In Act 3 of the play Martha Corey must tell the court and convince them that the allegation against her for being a witch is false.

She does this by giving her logic the way she can but is asked this bizarre question "How do you know, then, that you are not a witch?".

The fault in the question is that by probability one cannot know someone to be a witch by guess because witches do not exist or as they believed in their time were rare.

So, for a woman to be declared one, the court would need definite proof for it to be true.