Protoplasm is essentially uniform and similar in appearance and properties wherever found, whether in the tissues of the human body, in a blade of grass, or in the green slime of a stagnant pool. And yet probably no two samples of protoplasm are ever exactly similar in all respects, though we may never be able to detect their precise differences. These differences are due to the fact that the stuff is _alive_, and within it are constantly going on those changes accompanying metabolism, or the building up and tearing down processes that always accompany life. All separate masses of protoplasm, such as the one-celled amoeba or the individual cells of our own bodies, are constantly taking in food and as constantly throwing off wastes. Hence, in the very nature of things, it is impossible to find any mass of protoplasm absolutely pure. And a further and impassable barrier to chemical analysis, or indeed to any adequate scientific examination, lies in the fact that we can never deal with protoplasm exactly as it is, since no analysis can be performed upon it without destroying its life. And yet even dead protoplasm, and especially its most characteristic constituent, _proteid_, has been found the most difficult material in the world to analyze, and nobody as yet pretends to know its exact chemical make-up. The constant effort of natural science to press back the boundaries of the unknown is very liable to obscure some of the things most essential to any system of clear thinking regarding these matters. We are so prone to think that if only our microscopes were a little stronger, if only we could devise more effective methods of staining or of chemical analysis or chemical synthesis, we might really find out what life is, or what matter itself is; in short, that we might be able to solve in a scientific way the old, old riddle of existence. But already we have about reached the limits of the powers of the microscope; and even if we could devise a way of seeing the ultimate structures of which protoplasm is composed, how would we be any better off? Would we not have to attribute to each constituent of this living substance the properties which we now attribute to the whole?--that is, the properties which we attribute to masses of protoplasmic units, such as plants, or birds, or human beings?