 Houkes and Vermaas focus on the use plan formulated by designers as establishing the function of the artifact in the first instance. This is the predominant intentionalist element in their account. But on their view, this use plan must be supported by a justification that the plan will realize the function, and this requires knowledge of the causal roles of the physicochemical capacities of the artifact. Through this required justification the actual physical structure of the artifact constrains the intentions articulated in the use plan. This is a non-intentionalist element. In addition, they require a historical element in the form of the communication of the use plan from designer to user and subsequently from user to user. They refer to this element as “evolutionary”, in an apparent reference to cultural evolution, but clearly this element, too, is primarily intentional since the evolution is carried out in a series of intentional communications. Houkes and Vermaas refer to their theory as the ICE (Intention, Causal role, Evolution) theory of artifact function. Other mixed theories include those of Paul Griffiths (1993) and Philip Kitcher (1993).