ideational factors. Central to constructivism are the notions that ideas matter, and that agents are socially constructed (rather than given). Constructivist research is focused both on causal explanations for phenomena, as well as analyses of how things are constituted. In the study of national security, the emphasis is on the conditioning that culture and identity exert on security policies and related behaviors. Identities are necessary in order to ensure at least some minimal level of predictability and order. The object of the constructivist discourse can be conceived as the arrival, a fundamental factor in the field of international relations, of the recent debate on epistemology, the sociology of knowledge, the agent/structure relationship, and the ontological status of social facts. The notion that international relations are not only affected by power politics, but also by ideas, is shared by writers who describe themselves as constructivist theorists. According to this view, the fundamental structures of international politics are social rather than strictly material. This leads to social constructivists to argue that changes in the nature of social interaction between states can bring a fundamental shift towards greater international security. Challenging realism During constructivism's formative period neorealism was the dominant discourse of international relations, thus much of constructivism's initial theoretical work challenged basic neorealist assumptions. Neorealists are fundamentally causal structuralists, in that they hold that the majority of important content to international politics is explained by the structure of the international system, a position first advanced in Kenneth Waltz's Man, the State, and War and fully elucidated in his core text of neorealism, Theory of International Politics. Specifically, international politics is primarily determined by the fact that the international system is anarchic – it lacks any overarching authority, instead it is composed of units (states) which are formally equal – they are all sovereign over their own territory. Such anarchy, neorealists argue, forces States to act in certain ways, specifically, they can rely on no-one but themselves for security (they have to self-help). The way in which anarchy forces them to act in such ways, to defend their own self-interest in terms of power, neorealists argue, explains most of international politics. Because of this, neorealists tend to disregard explanations of international politics at the "unit" or "state" level. Kenneth Waltz attacked such a focus as being reductionist. Constructivism, particularly in the formative work of Wendt, challenges this assumption by showing that the causal