Because it attributes to material conditions the cause of social change - in opposition to previous idealist conceptions, particularly the idealism of Hegel, the German philosopher, which attracted Marx at one time, but whom he later claimed was standing on his head and had to be put right side up. Hegel claimed that the world was the working out of the idea. In other words that the idea came first and the real world was a reflection of the idea. Marx reversed this, pointing out that that the world was real and the idea was only a reflection of it in the head of man. Ideas do not float in the air or arise haphazardly in the brain of man. They arise out of definite material or economic circumstances.

Thus the roots of social change are to be found in the means and methods by which society gets its livelihood - by the production and distribution of the means to sustain life. Consequently the basis of all human life is the production and reproduction and reproduction of life and its needs. This is the overpowering influence in producing social ideas and changes in the prevailing ideas in the course of time.

In early communistic societies, and still where remnants of them still exist as in the Northern Territory of Australia, it was right to take what one needed of the communal food, use the communal tools, and have considerable freedom in sex relations in comparison with today. These societies were based on the family tie and not on property rights. Today if you walked into a shop and took a loaf because you were hungry you would find yourself in trouble, because with the development of property society the moral code changed. The moral rules became "Thou shalt not covet thy neighbours wife, nor his ox, nor his ass, nor anything that is his". As class society developed a state grew up to enforce these rules.

Among certain hunting peoples in the past infanticide was practised - it was accepted as morally right because it was necessary to keep down numbers owing to the uncertainties of hunting. The baby girls suffered most because the baby boys would grow into hunters and providers of food. This practice declined with the growth of settled agricultural communities because then the food supplies became more secure.

In ancient Egypt and Babylon a debtor sold himself into slavery to settle his debts, or he might sell his wife or children. It was considered morally right.

In the most advanced states of classical antiquity slavery was supported by by moralists and philosophers. The economies of Greece and Rome were largely built on the work of chattel slaves. Similarly it was accepted throughout America two hundred years ago. The Declaration of Independence and the Constitution of the United States are a sample of how sentiments can be at variance with acts when conditions rule acts.

The Declaration of Independence of 1776 laid down the following principles:
"We hold these truths to be self-evident, that all men are created equal, that they are endowed by their creator with certain inalienable rights, that among these are life, liberty and the pursuit of happiness."

When the Constitution of the United States was agreed to in 1787-9, section 9 contained the following:
The migration or importation of such persons as any of the states now existing shall think proper to admit, shall not be prohibited by the Congress. ….but a tax or duty may be imposed on such importation, not exceeding ten dollars for each person."

All men are equal - except those that are not. Section 9 was put in the economic interests of the southern slave states. At the same time the mass of the white population, the poorer section, was excluded from voting or framing the laws and regulations.

Amongst the Spartans of ancient Greece lying, cheating, and murder were virtues taught to the young Spartans. They were a relatively small group governing a large population of helots and slaves, whom it was essential that they keep in subjection.

In the early Middle Ages the lending of money at interest was looked upon as immoral because it cut across the close feudal self-supporting communities. Nowadays the lending of money is extolled because capitalism lays down the rules and the lending of money - or investment - is necessary capitalism.

In the Middle Ages generally women were in a subordinate position - little above that of a domestic slave. This was reflected in the views of some church dignitaries. Here is a selection:
St. Augustine "Why was woman created at all."
Tertullian: "Woman, thou art the devils gate, the betrayer of the tree, the first deserter of divine law. Marriage is not far from fornication."
St. Ambrose: "She is more fitted for bodily work … Remember that God took a rib out of Adam's body, not a part of his soul to make her. She was not made in the image of God like man."
St. Jerome: "Woman is the root of all evil."
St. Gregory: "It was no part of God's primitive design that the race should be continued by sexual union. Marriage is the outcome of sin."
It would be interesting to know how these saints proposed that the human race should be propagated. Presumably by miracles, like taking bones from men's ribs!

Much has been written from a moral standpoint about the destruction of the buffalo in America and its effect upon the Indian population. The action of the hide hunters and the suppliers of meat to the railway builders removed the main source of Indian subsistence, and, along with the activities of the fur hunters, was responsible for the bitterness and uprisings of the Indians. Without the slaughter of the buffalo to provide food for the railway builders, the railway link between East and West could not have been built at the time, and the economic surge forward as a result of it, would have been considerably delayed.

The Indian war dances of the latter part of the nineteenth century were appeals to the "Great Spirit" to drive the white man out, bring back the buffalo and the fishing territories to the Indians and, therefore the life they had enjoyed before the white man hemmed them in and destroyed it.

This is just another instance of the misery and turmoil capitalist economic development has brought to humanity in its pursuit of profit.

To sum up briefly. Behind all the ideals of the past and present since the beginning of written history are real men and women in the throes of the class struggle. Human beings are the prisoners of the ideals of their times; they read into moral principles the ruling economic ideas and needs of their time. But fortunately they can break out of prison.