The fundamental feature that characterizes living systems is autonomy, and any account of their organization as systems that can exist as individual unities must show what autonomy is as a phenomenon proper to them, and how it arises in their operation as such unities. Accordingly the following is proposed. (1) That autonomy in living systems is a feature of self-production (autopoiesis), and that a living system is properly characterized only as a network of processes of production of components that is continuously, and recursively, generated and realized as a concrete entity (unity) in the physical space, by the interactions of the same components that it produces as such a network. This organization I call the autopoietic organization, and any system that exhibits it is an autopoietic system in the space in which its components exist; in this sense living systems are autopoietic systems in the physical space. (2) That the basic consequence of the autopoietic organization is that everything that takes place in an autopoietic system is subordinated to the realization of its autopoiesis, otherwise it disintegrates. (3) That the fundamental feature that characterizes the nervous system is that it is a closed network of interacting neurons in which every state of neuronal activity generates other states of neuronal activity. Since the nervous system is a component subsystem in an autopoietic unity, it operates by generating states of relative neuronal activity that participate in the realization of the autopoiesis of the organism which it integrates. (4) That the autopoietic states that an organism adopts are determined by its structure (the structure of the nervous system included), and that the structure of the organism (including its nervous system) is at any instant the result of its evolutionary and ontogenic structural coupling with the medium in which it is autopoietic, obtained while the autopoiesis is realized. (5) That language arises as phenomenon proper to living systems from the reciprocal structural coupling of at least two organisms with nervous systems, and that self-consciousness arises as an individual phenomenon from the recursive structural coupling of an organism with language with its own structure through recursive self-description.