As many of you know, Niels Bohr found an understanding of quantum theory in his idea of 'complementarity.' To him, this was not an 'interpretation' of quantum theory, in the sense that it is something added on top of quantum theory whose validity is not on the same footing as the theory itself, but an essential part of quantum theory that is inseperable from the formalism. I first encountered Bohr several years ago, at a time when I was so frustrated with quantum theory almost to the point of madness because of the fact that I couldn't get an insight into quantum theory and what it means. Ever since I encountered Bohr, I had an almost messianic desire to expose other people to it, because it seemed so important. However I did not talk to anyone about it because it took a long time to read all his papers on this subject (several times) before I felt that I had a reasonable idea about what he was trying to say. I am starting this thread to hear what people think about complementarity. I found it convenient to choose an example that is not from physics, but from biology (described below). This is because the scope of the complementarity argument is far wider than physics, and is more appropriately viewed as a general lesson about the underlying conditions for the explanation of nature. To set the stage, I will briefly describe complementarity in physics. From the usual simplest examples of quantum phenomena, like the double slit experiment and the photoelectric effect, we see that the experimental data seem to have different aspects which are seemingly contradictory. For example, in the double slit experiment, the observable fact is a set of dots on the photographic plate. The individuality of the dots finds its expression in the concept of a particle, while the distribution of the dots finds its logical representation in the concept of wave. The trouble is that a particle picture cannot lead to a distribution of the form observed, so evidently we cannot use either picture without restriction if we are to avoid contradiction. The reciprocal restriction of the applicability of the particle or wave picture finds its expression in the uncertainty relations. Now I would like to describe an example from biology that Bohr gives, which as far as I know is not known to many people, other than people who have spent time with Bohr's original writings. Many people have the intuition that the characteristic features of living organisms seem to be outside the scope of a purely mechanical or physical description. The description of living organisms requires the use of the word 'purpose' which belongs to a frame of concepts which may be termed vitalistic. On the other hand, a system specified in a mechanical sense leaves no room for such a concept, and there does not seem to be any restriction on our ability to carry out such a description for life forms using microscopes etc. The apparent contradiction is resolved in the following way: A situation in which a life form exhibits its characteristic vitalistic behavior is not compatible with a simultaneous specification of its state in a mechanistic sense, and a situation in which such a mechanistic specification is carried out is incompatible with vitality, because such a situation will kill the life form. Thus the vitalistic and mechanistic descriptions are complementary and apply to mutually exclusive experimental situations.