We find that children, whose body is continually, as it were, in equilibrium, laugh or cry simply because they see others laughing or crying; moreover, they desire forthwith to imitate whatever they see others doing, and to possess themselves of whatever they conceive as delighting others: inasmuch as the images of things are, as we have said, modifications of the human body, or modes wherein the human body is affected and disposed by external causes to act in this or that manner.