The essential point of the positivist theory is no other source of knowledge except the straight and short way of perception through the senses. Positivism always holds trictly to that. Now, the two sentences: (1) there is a real outer world which exists independently of our act of knowing and (2) the real outer world is not directly knowable form together the cardinal hinge on which the whole structure of physical science turns. And yet there is a certain degree of contradiction between those two sentences. This fact discloses the presence of the irrational, or mystic, element which adheres to physical science as to every other branch of human knowledge. The effect of this is that a science is never in a position completely and exhaustively to solve the problem it has to face. We must accept that as a hard and fast, irrefutable fact, and this fact cannot be removed by a theory which restricts the scope of science at its very start. Therefore, we see the task of science arising before us as an incessant struggle toward a goal which will never be reached, because by its very nature it is unreachable. It is of a metaphysical character, and, as such, is always again and again beyond our achievement.
This is from Max Planck's discussion of the issue in Positivism and External Reality (for some reason I didn't put any further information in my notes; I think it was quoted in Jaki's The Road of Science and the Ways to God - in the Planck chapter, of course). This conception of science pervades his writings on the subject, as can easily be seen by looking at the Scientific Autobiography and The Philosophy of Physics. Consider the following passage from Scientific Autobiography:
In any case, we may say in summary that according to what exact natural science teaches us, the entire realm of nature, in which we human beings on our tiny mote of a planet play only an infinitesimally small part, is ruled by definite laws which are independent of the existence of thinking human beings; but these laws, insofar as they can at all be comprehended by our senses, can be given a formulation which is adapted for purposeful activity. Thus, natural science exhibits a rational world order the inner essence of which is and remains unknowable to us, since only our sense data (which can never be completely excluded) supply evidence for it. Nevertheless, the truly prolific results of natural-scientific research justify the conclusion that continuing efforts will at least keep bringing us progressively nearer to the inattainable goal, and they strengthen our inner hope for a constant advancement of our insight into the ways of the omnipotent Reason which rules over Nature. (tr. by Frank Gaynor, Greenwood Press (1971) pp. 181-182)
