实物单位是根据事物的自然属性和特点而采用的自然单位、度量衡单位、复合单位、标准实物单位。
真实变量：按实物单位衡量的变量。
那个时候，大多数的计量单位都是来源于实物、手、人体部位（比如“英尺”的英文原意为脚，英里则来源于拉丁语的milia passuum，意为1000步）或者工具（比如桶、杯子）。
Most were drawn from things to hand: the human body (the foot or the mile, which derives from the Latin milia passuum, or 1, 000 paces) and tools (barrels, cups).
它的本质就是人们可以下载信息单位，然后交易或者转让，这其实也是当前盈利性的实物交易系统中的一种形式。
Essentially people are able to download messaging units and trade or transfer them as part of what has become a lucrative barter system.
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