Why Chinese say "买东西"（buy east-west） instead of “买南北” （buy north-south）?

In the early of 1980s, when I was a senior in Peking University, in Beijing, China, I took a course called “Chemistry History”.  It talked about how the ancient Chinese made elixir, paper, fireworks, etc.  It was good for our Chemistry students to learn the development of chemistry.  It was an elective course, but the content was interesting, so most students enrolled.  It was held once a week, for 2 hours each time.
One day at the beginning of the course, the instructor said to us “When you meet someone, you may be asked ‘Where have you been?’.  You will answer: ‘I just went to buy some east-west’. Why do we say “买东西（buy east-west）”, but not “买南北（buy north-south）”？

He started to explain the story behind ‘east-west”.
In ancient China, people thought that the universe is made from five elements: 金木水火土.

In English, these are: Gold, Soil, Water, Fire, and Wood.  The five elements are located in five directions:  东西南北中. In English, these are: East, West, South, North, and Center.  Wood is in the East, Gold is in the West, Fire is in the South, Water is in the North, and Soil is in the Center, right under our feet.

However, Gold doesn’t mean gold only. It includes everything which is made from metals, or contains metals, such as a knife, car, gun, pan or plow.  Similarly, Wood includes everything which is made from wood, and everything from plants, such as furniture, houses, grains, vegetables and fruits.   In ancient times, all of these were merchandise, and people had to buy them.  But South, North, and Center, or in English, Fire, Water, and Soil were free at that time.  No one paid for those.  Therefore, people in ancient times bought “East-West”, not “North-South” and Center.

Even though Fire, Water, and Soil became valuable, and people eventually have to pay for, “买东西（buy east-west）” has become a standard phrase for Chinese.

We human beings have been buying “East-West” for thousands of years.  However, as I mentioned above, we need not only to buy “East-West” like cars made of metals; tables, chairs, fruits, vegetables and grains made of wood; we also have to buy “North-South” like gas, electricity, water, drink, etc., because the “North-South” products are not free any more.  Things are more expensive.  The most expensive thing is a house, which is built on the land and belongs to the Soil, or the Center.  Many people can afford “East-West” but can’t afford the “Center”.  If some ancient Chinese people had woken up today, they would have been shocked by the price of land.  Nowadays, all East-West-North-South-Center are valuable.  Nothing is free.

You will have all the “Gold-Wood-Water-Fire-Soil”, or, you bought all “East-West-North-South-Center”.  I have rented for years, no land, and no house. The American dream is for me is still a dream.

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