halt, they have always found means to advance further. But what
assurance have we that they will not one day come up against
impassable barriers? The experience of four hundred years, in which
the surface of nature has been successfully tapped, can hardly be
said to warrant conclusions as to the prospect of operations
extending over four hundred or four thousand centuries. Take biology
or astronomy. How can we be sure that some day progress may not come
to a dead pause, not because knowledge is exhausted, but because our
resources for investigation are exhausted--because, for instance,
scientific instruments have reached the limit of perfection beyond
which it is demonstrably impossible to improve them, or because (in
the case of astronomy) we come into the presence of forces of which,
unlike gravitation, we have no terrestrial experience? It is an
assumption, which cannot be verified, that we shall not soon reach a
point in our knowledge of nature beyond which the human intellect is
unqualified to pass.

But it is just this assumption which is the light and inspiration of
man's scientific research. For if the assumption is not true, it
means that he can never come within sight of the goal which is, in
the case of physical science, if not a complete knowledge of the
cosmos and the processes of nature, at least an immeasurably larger
and deeper knowledge than we at present possess.

Thus continuous progress in man's knowledge of his environment,
which is one of the chief conditions of general Progress, is a
hypothesis which may or may not be true. And if it is true, there
remains the further hypothesis of man's moral and social
"perfectibility," which rests on much less impressive evidence.
There is nothing to show that he may not reach, in his psychical and