Use your imagination and picture a market in the mid XV-th century in Europe. Farmers and tradesmen sell their products in the square of the city. Others want to buy and search for the best offer. At one point the buyer chooses one seller and buys, say wool. In exchange for the wool the buyer will give the seller a number of coins – money.
If we think about this transaction – wool for coins – we notice that it is a very visible one. The buyer takes the coins out of his bag (pocket) and gives them to the seller in exchange for the product.
Putting things a bit differently, for the buyer it is impossible to not see the coins (money) going out of his pocket and going into the seller’s hand and pocket.