The heart is more than a pump. Science is just now proving this; though, the Bible speaks of the heart about 1,000 times in its emotional, even thought originating role.
Here, Jesus specifically shows the fault-finding doubters that their ‘reasoning’ was happening “in their hearts”—in their innermost being.
Deep down must be clean too. Evil can originate from deep within. Understanding what’s going on there is a huge key to being free.
It’s Biblical to make sure our hearts are pure — not just our heads.
Mark 2:

1 When he [Jesus] entered again into Capernaum after some days, it was heard that he was in the house. 2 Immediately many were gathered together, so that there was no more room, not even around the door; and he spoke the word to them. 3 Four people came, carrying a paralytic to him. 4 When they could not come near to him for the crowd, they removed the roof where he was. When they had broken it up, they let down the mat that the paralytic was lying on.

5 Jesus, seeing their faith, said to the paralytic, “Son, your sins are forgiven you.” 6 But there were some of the scribes sitting there, and reasoning in their hearts, 7 “Why does this man speak blasphemies like that? Who can forgive sins but God alone?” 8 Immediately Jesus, perceiving in his spirit that they so reasoned within themselves, said to them, “Why do you reason these things in your hearts? 9 Which is easier, to tell the paralytic, ‘Your sins are forgiven;’ or to say, ‘Arise, and take up your bed, and walk?’

10 But that you may know that the Son of Man has authority on earth to forgive sins”–he said to the paralytic– 11 “I tell you, arise, take up your mat, and go to your house.” 12 He arose, and immediately took up the mat, and went out in front of them all; so that they were all amazed, and glorified God, saying, “We never saw anything like this!”