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