Why is it considered bad luck if you spill salt?

By an old superstition it's considered bad luck to spill salt. If one was to spill it in error they then must throw it over their left shoulder. Salt was considered very pure so throwing it over the left shoulder would temporarily stop the devil, therefore preventing any misfortune.

A few quotes ..."He is not worth his salt, is a common expression. It originated in ancient Greece where salt was traded for slaves."

"The first written reference to salt is found in the Book of Job, recorded about 2,250 BC. There are 31 other references to salt in the Bible, the most familiar probably being the story of Lot’s wife who was turned into a pillar of salt when she disobeyed the angels and looked back at the wicked city of Sodom."

" An old English belief has it that every grain of salt spilled represents future tears. The Germans believe that whoever spills salt arouses enmity, because it is thought to be the direct act of the devil, the peace disturber. The French throw a little spilled salt behind them in order to hit the devil in the eye, to temporarily prevent further mischief. In the United States, some people not only toss a pinch of spilled salt over the left shoulder, but crawl under the table and come out the opposite side."

"Jesus called his disciples "the Salt of the Earth." In Leonardo DaVinci's famous painting, "The Last Supper," Judas Escariot has just spilled a bowl of salt - a portent of evil and bad luck. To this day, the tradition endures that someone who spills salt should throw a pinch over his left shoulder to ward off any devils that may be lurking behind."

"In Buddhist tradition, salt repels evil spirits. That's why it's customary to throw salt over your shoulder before entering your house after a funeral: it scares off any evil spirits that may be clinging to your back."

"Shinto religion also uses salt to purify an area. Before sumo wrestlers enter the ring for a match - which is actually an elaborate Shinto rite - a handful of salt is thrown into the center to drive off malevolent spirits."

"Today, a gift of salt endures in India as a potent symbol of good luck and a reference to Mahatma Gandhi's liberation of India, which included a symbolic walk to the sea to gather tax-free salt for the nation's poor."

Easy to understand why the spilling of Salt was and still is considered to be bad luck...