...we cannot foresee which parts of our scientific knowledge may come to grief one day. Thus the belief in scientific certainty and in the authority of science is just wishful thinking: science is fallible, because science is human.

But the fallibility of our knowledge—or the thesis that all knowledge is guesswork, though some consists of guesses which have been most severely tested—must not be cited in support of scepticism or relativism. From the fact that we can err, and that a criterion of truth which might save us from error does not exist, it does not follow that the choice between theories is arbitrary, or non-rational: that we cannot learn, or get nearer to the truth: that our knowledge cannot grow.

— Karl Popper, The Open Society and Its Enemies

Though truth is not self-revealing (as Cartesians and Baconians thought), though certainty may be unattainable, the human situation with respect to knowledge is far from desperate. On the contrary, it is exhilarating: here we are, with the immensely difficult task before us of getting to know the beautiful world we live in, and ourselves; and fallible though we are we nevertheless find that our powers of understanding, surprisingly, are almost adequate for the task—more so than we ever dreamt in our wildest dreams. We really do learn from our mistakes, by trial and error. And at the same time we learn how little we know—as when, in climbing a mountain; every step upwards opens some new vista into the unknown, and new worlds unfold themselves of whose existence we knew nothing when we began our climb.

Thus we can learn, we can grow in knowledge, even if we can never know, that is, know for certain. Since we can learn, there is no reason for despair of reason; and since we can never know, there are no grounds here for smugness, or for conceit over the growth of our knowledge.

— Karl Popper, The Open Society and Its Enemies