This could also be translated as "You should not argue about this in your hearts" or "You should not doubt that I have the authority to forgive sins." (See: [[rc://en/ta/man/translate/figs-rquestion]])
This is an idiom referring to the part of a person that thinks or believes. Some languages will find it more natural not to include this phrase. (See: [[rc://en/ta/man/translate/figs-idiom]])
Jesus uses this rhetorical question to prepare them to connect his power to forgive with the miracle of healing that he is going to do. The whole question could be translated as "It is easy to say 'Your sins are forgiven,' but only God can cause this lame man to 'Get up and walk.'" (See: [[rc://en/ta/man/translate/figs-rquestion]])