Colloquial Language vs. Science: ‘Action’, ‘Reaction’, and ‘Interaction’

To begin with, let us become clear about what is meant by ‘action’ in this context. We are not talking about the behaviour of intentional Agents, i.e. our ‘actions’, nor are we talking generally about the motion of inanimate objects; we are only talking of the influence exerted by one object on another. The colloquial use of action is much broader. For instance, when a tennis player returns a ball across the net, we may describe everything the player does from the moment she initiates the swing until the racket connects with the ball as an ‘action’. However, if by ‘action’ we mean ‘influence exerted by one object upon another’, then the racket only begins to act on the ball when they become contiguous. We unreflec-tively think of the whole movement as an action upon the ball, because the swing is initiated and guided by the intention to strike the ball and because we think the power put into the entire swing is relevant for the outcome. The latter is in a sense true, but we have combined under one description what Salmon calls propagation and production.

Indeed, in an unreflective stance we are prone to think of the movement of a brick flying through the air, prior to its collision with a window, as a part of its ‘action’ upon the window. That is, as if that previous motion involves a similar kind of effort as rhe muscle work of the tennis player. However, prior to actually touching the window, the brick exerts no influence upon it and exerts no effort to move through the air. Unaccelerated motion is not an action according to the law of inertia, which says that material bodies continue in their motion in the absence of forces. A moving object, unaffected by anything else, continues in its motion without having to ‘do’ anything about it. Uniform motion is really a state, a state of motion. We may say that the brick we threw ‘flies through the air’, as if it was actively engaged in some activity, but of course this is just a figure of speech. The brick only acts on the window in so far as it is exerting an influence on it, and that influence first begins on contact and ends as soon as they lose contact.

As already mentioned in Section 3.10, the above-mentioned ambiguity about action is actually found in the jargon of classical mechanics, not just in ordinary language. The term ‘action’ is used to denote the exertion of force in interactions between objects, and for the time integral of the kinetic energy of a material object, as taken between two times, whether or not that object exerts a force on anything. This may allow talk of uniform motion as an ‘action’. Hertz objected to this use of the term (1956: 228), but with no noticeable effect. It is still used to denote the sum of the kinetic energy of an object. However, his objection is still valid and here I use the term ‘action’ only to denote the exertion of influence between objects in the former sense.

Let us turn to ‘interaction’ and ‘reaction’. In colloquial use, ‘interaction’ refers to various types of communication where the parts take turns at affecting each other, e.g. in conversation when one person greets the other and the other ‘reacts’ with an appropriate response. Or, to use a somewhat construed example of communication, when boxers take turns delivering blows. This is not the kind of interaction I am talking about here. I am talking about the mutual exertion of influence between two objects as they come into connection with each other; when brick hits window, or glove meets face. The third law states that the window acts on the brick with a force of the same magnitude as the brick acts on the window, and the same goes for the face exerting a force on the gloved hand.

As for ‘reaction’, then it is popularly used to refer to the ensuing response to some influence, e.g. when someone ‘reacts’ to a greeting by greeting back or reacts to a blow to the head by wincing. In this use, ‘reaction’ means ‘effect’ or ‘response’. In physics ‘reaction’ refers to the resistance immediately offered by a thing when it encounters another thing, e.g. the resistance offered by the window being hit. That is, ‘reaction’ does not refer to the breaking of the window when it is hit by a brick; the breaking is an effect. It is the resistance offered by the window before, or while, it breaks that is a reaction. In the same way, wincing from a blow to the head is not a reaction but an effect. The immediate resistance offered by the face to the glove is a reaction, and it is felt in the hand of the boxer and may well feel as painful as the blow to the head, say, if a bone breaks in the hand on impact.