There is an intimate link between teaching as didactical practice and selection (Hopmann, s.a., p. 148-149). It is neither self-evident what to educate for, nor which content, methods and technologies will promote the intended learning outcomes among students with different qualifications and different interests. 

During the last 10 years, we have witnessed a growing societal interest in the education system. According to A. Hargreaves (2003) this is linked to the emergence of the knowledge society and an enhanced focus on innovation and creativity. Knowledge is considered as a fundamental resource for future development and sustainability. This has, among other things, led to the highest participation rates ever in higher education and enhanced diversity in student population that questions the main mission of higher education and modes of delivery (Biggs & Tang, 2011, p. 3).

The enhanced interest in the education system has led to a growing focus on assessment and documentation of quality in teaching and learning at all levels at the education system, including an increased demand on teachers to substantiate their didactic decisions (Alexander, 2000; Biggs & Tang, 2011; Hopmann, 2008; Jongbloed, Enders, & Salarno, 2008; Ozga, Dahler-Larsen, & Segerholm, 2011). The idea of accountability has taken different forms across education systems (Hopmann, 2008; Labaree, 2012), but the societal interest in schooling cannot be disregarded. 

In short, we might conclude that today's teachers face internal as well as external challenges, which require a reiterated 'relating to themselves'. Teachers must be able to take a second-order perspective on themselves and their selections, i.e. to observe their selections as selections in order to answer, whomever might ask: "Why did you select this instead of something else?" (Hopmann, s.a., p. 144). 

An important question is, where teachers find the arguments for the span of selections they make in planning and evaluation of teaching and learning. The aim of this paper is twofold: To introduce a systematic distinction between three types of knowledge: Experiential didactic knowledge, didactics and science of teaching, and to demonstrate how they in different ways provide teachers with knowledge that can be used in selection and reasoning.

The theoretical frame for the construction of different types of knowledge for didactic selection and reasoning is found in second-order systems theory as described by the German sociologist Niklas Luhmann. Luhmann describes the modern society as functionally differentiated (Luhmann, 1990, 1995, 2012). The underlying assumption is that society over time responds to enhanced complexity by functional differentiation that is, differentiation into societal sub-systems such as economics, law, science and education that handle a specific aspect of the total societal communication. The function of a sub-system is expressed in the coding of the system's commucation, i.e. a system specific code that conditions operations of the functional system. For instance, the commucation of the economic system is organized around the code gain/loss; the communication of law around the code legal/unlegal, and with special interest for the paper, the communication of the scientific system is organized around the code true new knowledge/false new knowledge and the communication of the education system around the code better knowledge/worse knowledge (Luhmann, 2012). In his analysis of the society's education system, Luhmann uses the distinction between the scientific and the educational code to develop the concept of reflection theories. Didactics serves as reflection theory for teaching and upbringing in the sense that it identifies itself with the objectives and institutions of the education system (Luhmann, 2002, p. 201). Similar obligations to the function and codes of the education system cannot be found in the science of teaching.

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