Living systems are characterized as self-generating and self-maintaining systems. This type of characterization allows integration of a wide variety of detailed knowledge in biology. The paper clarifies general notions such as processes, systems, and interactions. Basic properties of self-generating systems, i.e. systems which produce their own parts and hence themselves, are discussed and exemplified. This makes possible a clear distinction between living beings and ordinary machines. Stronger conditions are summarized under the concept of self-maintenance as an almost unique character of living systems. Finally, we discuss the far-reaching consequences that the principles of self-generation and self-maintenance have for the organization, structure, function, and evolution of singleand multi-cellular organisms.

Excerpt: In this paper, we will discuss the questions whether a unified concept of the world is possible and whether ontological gaps exist in the world that would make such a view impossible. We will discuss the problems mainly with respect to biology, because biological systems are systems which, on the one hand, are strongly bound to physics and chemistry, but at the same time extend into the domains of psychology and social sciences, at least with respect to human beings. Thus, biology is central to any discussion about the unity of the world.

Excerpt: First I shall sketch some elements of a general systems theory as a framework in which steps of unification may take place. Since our world is historical it must be a theory of dynamic systems. Then I shall discuss the program of microreduction, which is the most prominent version of reductionism. I will argue that the experiences from physics speak strongly against that program.

The way physics and other parts of science work can be explained in the framework of radical constructivism. However, this constructivist view itself shows that a uniquily accepted epistemology, constructivism or any other, would not be an advantage for the development of science. Unlike physics some parts of science successfully use constructivist concepts inside their theories. Because this is the case particularly in learning theory, constructivist ideas can help to improve physics teaching.

Key words: consensuality, language-games, quantum theory, realism in physics, scientific education

The concepts of self-generation, autonomous boundary and self-maintenance are explained briefly. The “protocell” is presented as a model of self-maintenance which is based on simple physical mechanisms of diffusion and reaction. The time evolution of the surface of the protocell is taken into account explicitly in the form of a Stefan condition giving rise to a non-linear feedback of the surface motion to the reaction and diffusion processes inside the protocell. The spatio-temporal dynamics are investigated, particularly in the neighbourhood of the stationary states, showing a self-maintaining behaviour under a certain range of nutritional conditions. Under another set of conditions we find an instability leading to a division process so that the population of protocells becomes self-maintaining instead of the single individual. The presented formulation of the protocell model is crucially improved compared with a previous version which required boundary conditions at infinity. The previous version was not strictly self-maintaining since dynamics outside the cell were essential for its behaviour.

Key words: self-maintenance, autopoiesis, morphogenesis, protocells, division model.