Newton's third law is not something which students easily accept. This is largely due to the widespread notion that the force exerted by a moving body is directly related to the speed or to the mass of the body (or to both) and not to the nature of any interaction in which that body might be involved. The aim of the paper is to identify some of the issues upon which debate took place over the phenomenon of impact in the early 18th Century. This is achieved by studying a number of the popular textbooks and commentaries on mechanics which were published at that time. Some implications for teaching Newtonian mechanics are outlined.