Transforming the body-only system into the body-plus-tool system

The traditional definitions of tool use typically operationalize the functionality of an object and the effect of its use on the environment and, typically, do not account for the dynamic relation among the body, task and environment that result in actions with a tool. This omission severely restricts the utility of these definitions for comparisons of tool use behaviour across individuals, populations and species. To address this issue, we propose an embodied theory of tool use based on the premise that a tool transforms the body-only system into the body-plus-tool system. It (1) explains the development of a tool use behaviour in terms of constraints on the development of tool use movements imposed by different features of the body, task and environment, (2) measures the dexterity of an actor in terms of the spatiotemporal organization of tool use movements that optimize at least one composite performance outcome variable and (3) measures the complexity in the use of a tool in terms of the control of the biomechanical degrees of freedom of the body-plus-tool system. Such an embodied theory of tool use is applicable across species, tasks and environments.