To what extent and in what respect is science intellectually valuable? This is a controversial matter. What is hardly disputed is that what is alterable in science is of mere ephemeral value; and what is valuable in it is that which is more universal and permanent, that which is more solid and lasting. One of the very few philosophers who oppose this accepted view is Sir Karl Popper. In his view, science is so valuable because of its open- mindedness, because any of its achievements may at any time be given up and newer achievements may be hoped for to replace the relinquished ones. Science, says Popper, is at constant war with itself, and it progresses by revolutions and internal conflicts.