"The middle of the fourth millennium B.C. is the period at which Neolithic society in the great eastern river valleys developed the first form of civilization. ...For understanding how the impressive technological achievements of the early civilizations were brought about, it is important to notice that, even when the tiller of the soil was regarded as a free man rather than as a serf, forced labor on public works was commonly exacted (in Egypt it was still customary when de Lesseps began to dig the Suez Canal)... we may trace from this period the growth of slavery, which had its origin at the stage when it first paid to keep captives alive because their work was worth more than their keep. Every war produced its quota of human plunder; traders, too, dealt rapidly in a commodity that could carry itself to market; and within the community enslavement was commonly the fate of the criminal and the debtor. Slaves of the god or of his temple and of the king or of his state provided a numerous body of workers, sometimes well trained and often cruelly exploited, while throughout ancient history almost every reference to agriculture or industry in civilized countries must be taken to imply the employment of slaves, usually in small numbers, alongside farmers and craftsmen enjoying different degrees of personal freedom."