One of the main functions of legal reasoning is to determine the truth-value of legal statements, i.e., of statements that assert that certain actions are legally required or permitted. In general, the notion of legal system provides a useful framework, which allows us to distinguish between legal norms and other socially relevant norms. Two concepts of legal system are often employed in legal discourse. On the one hand, it is usually claimed that the material provided by a certain authority has systematic structure, i.e. the norms are structured by means of specific relationships. e.g. hierarchical relationships. In this sense, law is an institutional system. On the other hand, it is frequently asserted that legal norms can be reconstructed systematically starting from the notion of logical consequence. In this sense, law can be regarded as a deductive system. Legal reasoning, then, would use in an explicit or implicit way both notions of normative system in the process of establishing the truth conditions of legal statements. However, this traditional conception of legal reasoning and normative systems has been object of important challenges. In this paper. I consider two groups of arguments against the systematic reconstruction of law: 1) The Institutional Conception and the force of legal norms. It is necessary to recognize an important difference between the fact that a norm regulates an action and the fact that a norm is legally obligatory (i.e., it can justify an institutional decision). Even if it were true that all the norms that belong to a system (and only them) were legally obligatory this would be a contingent connection. That means that the reduction of legal statements to mere "membership statements' would be a fallacy. According to this reconstruction, jurists should abandon the task of a systematic reconstruction of law, and they should only deal with the justificatory dimension of law. b) The Deductive Conception and the defeasible nature of legal reasoning. The systematic reconstruction of law usually requires admitting the normative relevance of norms that are logical consequences of some other valid norms. In this respect, law could be reconstructed as a set of norms, closed under the notion of logical consequence. However, it is often claimed that legal reasoning is not a deductive one. On the contrary. legal reasoning would give relevance to implicit circumstances that defeat the general solution provided by general norms of legal systems. According to this reconstruction, to grasp the true nature of legal norms is incompatible with the systematization of law. In other words: a systematic reconstruction of law prevents us from understanding the defeasible nature of legal norms and the role they play in a justificatory argument.Finally, after analyzing these challenges, I show to what extent a systematic reconstruction of law is required in order to provide a sound analysis of both the institutional force of a legal norm and its defeasible nature. My conclusions are that: (i) to identify a norm which can justify an institutional decision is logically dependent on a previous identification of norms that are members of a particular legal system, and (ii) that in order to understand the relevance of contextual elements, which can defeat a legal norm, it is necessary to determine the scope of such a norm, a feature that largely depends on its logical consequences.