Works on System thinking are in progress within the research unit in the Catholic University of Milan. The group publishes here 5 seminars held in winter 2010-2011. During the discussions the attention has been focused on many systemic questions. I here take into account three topics: The definition of “system”. The meaning of the word “system” cannot be both precise and complete; it depends on and is determined by the different disciplines that make use of it. The knowledge of systems. A systemic approach in epistemology is preferable to an analytical one because both to the human phenomenological knowledge and to the sciences the world looks as a set of individual objects. The ontology of systems. A system is an object having properties that its parts don’t have. This fact suggests that the new properties originate from the relations between parts, not from the parts.