For the advance might at some point be arrested by an insuperable wall.

Take the particular case of knowledge, as to which it is generally taken for granted that the continuity of progress in the future depends altogether on the continuity of human effort (assuming that human brains do not degenerate).

This assumption is based on a strictly limited experience.

Science has been advancing without interruption during the last three or four hundred years; every new discovery has led to new problems and new methods of solution, and opened up new fields for exploration.

Hitherto men of science have not been compelled to halt, they have always found means to advance further.