“But there were certain of the scribes sitting there, and reasoning in their hearts,”
Now some teachers of the law were sitting there, thinking to themselves — the teachers of the law (scribes) are present in the house, observing, and their reaction to Jesus' forgiveness declaration is internal. They have not spoken yet; their objection is in the heart, not on the lips. Mark's description of their inner dialogue is either a theological claim (Jesus perceived their thoughts) or a narrative technique for presenting the inevitable objection that his readership would also raise. The presence of scribes in the house is a new element in the narrative — the Galilean religious establishment is now monitoring Jesus, and the conflict that will define the rest of the Gospel has begun.
Church Fathers on Mark 2:6
Because the compassion of God deserts not even carnal persons, He accords to them the grace of His presence, by which even they may be made spiritual. After the desert, the Lord returns into the city.
But Matthew writes this miracle as if it were done in the city of the Lord, whilst Mark places it in Capernaum, which would be more difficult of solution, if Matthew had also named Nazareth. But seeing that Galilee itself might be called the city of the Lord, who can doubt but that the Lord did these things in His own city, since He did them in Capernaum, a city of Galilee; particularly as Capernaum was of such importance in Galilee as to be called its metropolis?
He saw the faith of the sick man himself, since he would not have allowed himself to be carried, unless he’d had faith to be healed.