Question:What is sin? Is it related to the spiritual or the physical?
The simplest definition of sin is, "Whoever commits sin also commits lawlessness, and sin is lawlessness" (I John 3:4). Sin is when you break one of God's laws.
All sins start from inside, "For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies. These are the things which defile a man" (Matthew 16:19-20). The body is used by the spirit of a man often to commit the actual acts of sin. But the decision break a law of God first starts in the spirit.
The temptation to sin can start from the body's desires. Temptation is not the same is sin. Temptation is being placed in a situation where you believe that to achieve what you desire you must break a law of God. Satan often uses our physical desires as a way to distract us. We're so busy wanting something that we never look to see the hidden sin that comes in the act of getting. "Let no one say when he is tempted, "I am tempted by God"; for God cannot be tempted by evil, nor does He Himself tempt anyone. But each one is tempted when he is drawn away by his own desires and enticed. Then, when desire has conceived, it gives birth to sin; and sin, when it is full-grown, brings forth death. Do not be deceived, my beloved brethren" (James 1:13-16).