The superior insight of Marx is shown in the very first sentence of his great work. The careful reader at once perceives that the first paragraph of the book strikes a keynote which distinguishes it from all other economic works comparable to it in importance. Marx was a great master of the art of luminous and exact definition, and nowhere is this more strikingly shown than in this opening sentence of "Capital": "The wealth of those societies _in which the capitalist mode of production prevails_ presents itself as an immense accumulation of commodities, its unit being a single commodity."[155] In this simple, lucid sentence the theory of social evolution is clearly implied. The author repudiates, by implication, the idea that it is possible to lay down universal or eternal laws, and limits himself to the exploration of the phenomena appearing in a certain stage of historical development. We are not to have another abstract economic man with a world of abstractions all his own; lone, shipwrecked mariners upon barren islands, imaginary communities nicely adapted for demonstration purposes in college class rooms, and all the other stage properties of the political economists, are to be entirely discarded. Our author does not propose to give us a set of principles by which we shall be able to understand and explain the phenomena of human society at all times and in all places--the Israel of the Mosaic Age, the nomadic life of Arab tribes, Europe in the Middle Ages, and England in the nineteenth century. In effect, the passage under consideration says: "Political economy is the study of the principles and laws governing the production and distribution of wealth. Because of the fact that in the progress of society different systems of wealth production and exchange, and different concepts of wealth, prevail at different times, and at various places at the same time, we cannot formulate any laws which will apply to all times and all places. We must choose for examination and study a certain form of production, representing a particular stage of historical development, and be careful not to attempt to apply any of its laws to other forms of production, representing other stages of development. We might have chosen to investigate the laws which governed the production of wealth in the ancient Babylonian Empire, or in Mediaeval Europe, had we so desired, but we have chosen instead the period in which we live."