Humans can behave adaptively in the different dynamical conditions. In order to realize the task quickly, smoothly or efficiently under the various environments, it is required to adjust the kinematic and dynamic relations among the body and environment according to the task. Recent studies have shown that humans can acquire a neural representation of the relation between motor command and movement, i.e. learn an internal model of the environment dynamics. Then, we can compensate for the external dynamic perturbation in a feedforward manner. Considering the situation of performing the complex tasks, such as using the tools, it is necessary to perceive a simple dynamics from complicated ones. The present paper discusses whether humans can identify one side of dynamics from the mixed environment dynamics.