The physical forces of nature are limited. The earth is endowed with a fixed quantity of materials that we call gaseous, liquid and solid. It receives a certain amount of heat from the sun which, for all practical purposes, may be considered a fixed quantity of energy, even if in eons it may be exhausted. The physical energy of man is devoted to the transformation of these physical forces under the law of conservation; he can neither add to nor diminish the quantity. He can transform solid into gas and gas into liquid. He can, according to common speech, consume some of these products, but his consumption is only another transformation. His own body is but one of the forms of physical energy on the way toward another form. These elements of nature, formerly limited to earth, air and water, are now listed under many titles of what are called elements; I believe over sixty that have not yet been differentiated, but all may yet be resolved into a unit of force. You will observe that in our arithmetic we have ten numerals which can be divided into fractions. In our music we deal with seven notes and their variants. In our alphabet we have twenty-six letters. These factors correspond in some measure to what we call elements in nature. There is a limit to the number of combinations that can be made of the numerals and their fractions, to the notes of music and their variants, and of the letters of the alphabet; but in each case this limit is so remote as to be negligible, like the exhaustion of the heat of the sun. May we not deal with the elements of nature in the same way? Can any one prescribe a limit to their conversion and reconversion to the use of mankind? Is it not in these processes of conversion that we derive our subsistence? We make nothing. All that we can do is to move something. We move the soil and we move the seed; nature gives the harvest. We direct the currents of falling water, of heat and of steam; nature imparts the force or energy to which man has only given a new direction. We are now imparting new directions to the force that we call electricity, and to what we call cold. What is the force from which we derive this power of transforming physical energy? May we not call it mental energy? Is not mental energy the factor in mankind by which he is differentiated from the beast? Does not man only accumulate experience, and is there any limit to the power of mind over matter? If these points are well taken, mental energy is the fourth and paramount factor in providing for material existence, and the science of political economy, which deals with land, labor and capital, becomes a purely metaphysical science when we admit the force of mental energy into the combination.