Matthew 5.29 - 30
If your right eye causes you to sin, pluck it out and throw it away. It is better that you lose one of your members than that your whole body is thrown into Hell. And if your right hand causes you to sin, cut it off and throw it away; it is better that you lose one of your members than that your whole body go into Hell.
Okay, it's clearly neither your physical hand or eye (or foot, in Mark's version of this) that's causing you to sin. Your way of seeing, your favorite means of getting results, your methods of getting from one place (figurative or literal) to another might need to go.
And what is it to 'sin'?
Our guy is speaking to a Jewish crowd who live in The Promised Land, but haven't found it the land of freedom and blessings their ancestors had been promised.
What they want is to live in the Kingdom of God, that land as it's meant to be when God reigns over it, as Jesus has been saying is starting to manifest. 'Sin' has been their accepted explanation for why conditions around them have always been unsatisfactory and literally unacceptable. 'Sins' are the obstacles to living under God's explicit jurisdiction, blameless and blessed.