HOW CAN WE "SIN?"

HOW CAN WE "SIN?" If day follows night, faithfully; If the seasons flow in their prescribed order, inexorably; If our solar system does the same; if likewise does our spiraling Milky Way galaxy; then, who are we, the veritable least of these, to deem ourselves to be capable of free will? To have the capacity to do anything other than what we are supposed to do? In short: to ever possibly sin?