The idea in conceptual metaphor theory that when a target domain is structured in terms of a particular source domain, this highlights certain aspects of the target while simultaneously hiding other aspects. For example, invoking the metaphor argument is war highlights the adversarial nature of argument but hides the fact that argument often involves an ordered and organised development of a particular topic (He won the argument, I couldn't defend that point, and so on). In contrast, the metaphor an argument is a journey highlights the progressive and organisational aspects of arguments while hiding the confrontational aspects (We'll proceed in step-bystep fashion; We've covered a lot of ground).

H hierarchy of domain complexity A theoretical construct in cognitive grammar. Concepts presuppose the domains against which they are understood, leading ultimately to domains that do not presuppose anything else. This gives rise to a hierarchy of domain complexity in which a domain lower in the hierarchy is presupposed by the domain(s) higher up. For instance, consider the lexical item knuckle. This profiles a concept that is understood with respect to the domain hand. In turn, the domain hand is understood with respect to the domain arm; arm is understood with respect to the domain body, and body is understood more generally in terms of (three-dimensional) space.

However, it is difficult to envisage another domain in terms of which we understand space. This follows as space is a basic domain : one that derives directly from sensory experience of the world, such as visual perception and our experience of motion and touch. Thus space is not understood in terms of a further conceptual domain but is directly grounded in terms of embodied experience.