PB At what point does the transition occur from the non-path to the path in a biological system? Is a DNA molecule already a classical object, or is a cell a classical object? Heisenberg: There is, of course, not a very well defined boundary; it is a continuous change. When we get to these very small dimensions we must be prepared for limitations. I could not suggest any well-defined point where I have to give up the use of a word. It's like the word mixing in the story; you cannot say 'when I have two things, then I can mix them.' But what if you have five or ten? Can you mix then? PB It seems to me that there is something very important here about language. We are living beings formed from coherent structures like DNA and we apparently have classical paths and our existence is understandable within this language. But then we can analyse by reducing these complex, coherent wholes to smaller and smaller parts, and is it nor perhaps this process of reduction that is at the root of the paradox? Heisenberg: I would say that the root of the difficulty is the fact that our language is formed from our continuous exchange with the outer world. We are a part of this world, and that we have a language is a primary fact of our life. This language is made so that in daily life we get along with the world, it cannot be made so that, in such extreme situations as atomic physics, or distant stars, it is equally suited. This would be asking too much. PB Is there a fundamental level of reality? Heisenberg: That is just the point; I do not know what the words fundamental reality mean. They are taken from our daily life situation where they have a good meaning, but when we use such terms we are usually extrapolating from our daily lives into an area very remote from it, where we cannot expect the words to have a meaning. This is perhaps one of the fundamental difficulties of philosophy: that our thinking hangs in the language. Anyway, we are forced to use the words so far as we can; we try to extend their use to the utmost, and then we get into situations in which they have no meaning. DP In discussing the 'collapse of the wave function' you introduced the notion of potentiality. Would you elaborate on this idea? Heisenberg: The question is: 'What does a wave function actually describe?' In old physics, the mathematical scheme described a system as it was, there in space and time. One could call this an objective description of the system. But in quantum theory the wave function cannot be called a description of an objective system, but rather a description of observational situations. When we have a wave function, we cannot yet know what will happen in an experiment; we must also know the experimental arrangement. When we have the wave function and the experimental arrangement for the special case considered, only then can we make predictions. So, in that sense, I like to call the wave function a description of the potentialities of the system. DP Then the interaction with the apparatus would be a potentiality coming into actuality? Heisenberg: Yes. DP May I ask you about the Kantian notion of the 'a priori' an idea which you introduced, in a modified sense, into your discussions of quantum theory. Heisenberg: As I understand the idea of 'a priori,' it stresses the point that our knowledge is not simply empirical, that is, derived from information obtained from the outer world through the senses and changed into data in the content of our brain. Rather, 'a priori' means that experience is only possible when we already have some concepts which are the precondition of experience. Without these concepts (for instance, the concepts of space and time in Kant's philosophy), we would not even be able to speak about experience. Kant made the point that our experience has two sources: one source is the outer world (that is, the information received by the senses), and the other is the existence of concepts by which we can talk about these experiences. This idea is also borne out in quantum theory. PB But these concepts are pary of the world also. Heisenberg:Whether they belong to the world, that is hard to say; we can say that they belong to our way of dealing with the world. PB But we belong to the world, so, in a sense, these activities of ours also belong to the world. Heisenberg: In that sense, yes. DP You modified the 'a priori' by introducing it as a limited concept, is that true? Heisenberg: Of course, Kant would have taken the 'a priori' as something more absolute than we would do in quantum theory. For instance, Kant would perhaps have said that Euclidean geometry would be a necessary basis for describing the world, while we, after relativity, would say that we need not necessarily use Euclidean geometry; we can use Riemannian geometry, etc. In the same way, causality was taken by Kant as a condition for science. He says that if we cannot conclude from some fact that something must have been before this fact, then we do not know anything, and we cannot make observations, because every observation supposes that there is a causal chain connecting that which we immediately experience to that which has happened. If this causal chain does not exist, then we do not know what we have observed, says Kant. Quantum theory does not agree with this idea, and in fact proves that we can even work in cases where this causal chain does not exist.