When addressing the question of stability and of instability a philosopher may be talking on two very different planes or levels, as it were. The deep and the superficial, or common-sense. (Strictly, the superficial may be different from the common-sense, but let us ignore this.) He may speak superficially or common-sensically of relative stability and instability, and he may speak metaphysically of stability inherent in things he considers, of ultimate stability, as it were, or of ultimate instability. It is my wish to stress that when I speak of the stable and the unstable in science I speak on the superficial or common-sense level, not on the metaphysical level. The metaphysical situation concerning science is rather obvious — or at least has been so since Einstein: science, like all human endeavour, is essentially or inherently unstable; and this makes the discussion of its possible stability on the metaphysical level rather a bore. It is much more interesting to ignore these inherent qualities on the metaphysical level and speak superficially on the common-sense level of what is more stable and what is more ephemeral in science, and why.