Step 1: Break the Latin maxim.
Actus = act; non facit = does not make; reum = guilty (accused); nisi = unless; mens sit rea = the mind is also guilty.
Step 2: Doctrine of mens rea.
This maxim expresses the principle that criminal liability requires both a guilty act (actus reus) and a guilty mind (mens rea). An act alone, without criminal intent, is generally not enough for conviction.
Step 3: Select the correct meaning.
The act does not make a person guilty unless the mind is also guilty, emphasizing that intent is essential to criminal liability.
\[ \boxed{\text{The act does not make a person guilty unless the mind is also guilty}} \]