From Rav Weinberger's Shabbos shiur this morning:
Rav Yisroel Salanter in Igeres Hamussar: "ha'adam asur b'muskalo, v'chofshi b'dimyono." "Man is imprisoned by his intellect and free in his imagination."
Rav Weinberger's explanation: A person's intellect and soul, to whom G-d's reality is blazingly obvious, one is bound ("asur") to do what is right and avoid what is wrong. To the intellect/soul, one can no more look at an inappropriate image than he could stick his hand in a fire or cut himself with a knife.
But the force of the imagination/fantasy/body makes a person feel "chofshi b'artzeinu," free to do right or do wrong. But that feeling is pure dimyon, fantasy.
"Asur b'muskalo" is why a person says he "can't" eat on Yom Kippur, and "chofshi b'dimyono" is why he feels it's not so pashut that you can't conceal income on his tax return.
IY"H may we all be zoche to attain the feeling of "I can't" when it comes to doing anything against Hashem's will!