In logic, the term extension refers to the set of objects that share some defining characteristics. It is distinguished from the complementary term intension, which is the set of defining characteristics those objects have in common. Extension refers to objects of thought or consciousness, including physical objects (it also therefore refers to the extension in space of such objects). Intension refers to properties a thing must possess so that a particular term can be applied to it. Indirectly, it implies an agent assigning properties to objects, and the operations of applying them. (The term intention in logic has a similar meaning: "an instrument, such as a concept, for knowing and referring to a thing as it exists in the mind").
Objects and events in the environment have extension in space, and also in the logical sense of being the recipients of qualities assigned to them by an agent for whom they mean something. If this agent is an organism, they have significance for its well-being and survival which the organism recognizes and upon which it acts. Meaning cannot be accounted for by physical causes, even though it may refer to them. It is altogether another form of description-- in terms of logical relationships rather than physical ones. Cause can account for the interactions within a system extended in space, all from the point of view of an outside observer. Only mind-- an intentional system-- can confer meaning, and so explain what goes on within the observer as subject rather than as object observed. Intentionality is what gives such a system the power to look outward at the extensional world. The difference between extension and intension --or intention, as it will be used here-- is the difference of perspectives which is the essence of the Mind-Body Problem. One is the perspective on events in physical space that is presented in phenomenal experience. The other is the perspective of (or from) the logical events going on within the cognitive agent whereby that phenomenal experience arises. The operations within a brain can be described extensionally, since they take place in physical space and time, and explained as a causal sequence leading to behavior. But the experience of the subject can only be understood intentionally.
In Aristotle's system there were four types of cause, each serving a complementary aspect of what explanation meant to the Greeks. The material cause of a change of state was the presence of a medium in which the change takes place. An efficient cause was some agent or preceding event that brings about the change. Its formal cause was its tendency or end state. Its final cause was its purpose, use, or reason for being. The first three types of cause are extensional; they describe physical operations in space and time. But the notion of final cause implies logical operations having nothing to do with space and time, and everything to do with intention.
According to Berkeley, Hume, and some later
philosophers, causality amounts to nothing more than a succession in time.
One event is regularly seen to follow another, and from this we understand
that one is the cause of the other. But cause intuitively seems to involve
power of one thing over another as well as a mere succession of events.
Hume criticizes this idea of "necessary connection":
"When we look about us toward external objects, and consider the operation of causes, we are never able... to discover any power or necessary connexion; any quality which binds the effect to the cause, and renders the one an infallible consequence of the other... In reality, there is no part of matter, that does ever, by its sensible qualities... give us the grounds to imagine, that it could produce any thing, or be followed by any other object, which we could denominate its effect..."
Hume continues by arguing that we get this idea of power from our early experience of voluntary movement-- an idea expanded upon by Piaget. We learn to project the experience of power over our own bodies onto inanimate matter. Causality, in other words, is something projected into experience by the mind. It begins with the infant's noticing correlations among bodily sensations, visual impressions, etc. The baby learns it can "cause" its limbs to move by discovering that they do move in conjunction with certain sensations. Similarly, it learns it can indirectly cause the movements of other objects. But volition involves more than the mere association of sensations. It entails a concept of self, and a concept of a world on which the self can act, and from which it is felt to be distinct. Cause and world arise together as notions, when objects are recognized as entities mutually interacting and distinct from self. Causality is intimately bound up with the separation of subject and object.
The process, engaged in by the sensory mind, of evoking an interaction among external objects to organize the flux of sensory patterns, has its intellectual counterpart in the scientific quest for entities and causal explanations of the laws of nature. The finding of regularities in nature gives the power of prediction, especially if they can be quantified. Over and beyond that, we still want to know what sense can be made of these patterns. Must there not be an orderly reality underlying and giving rise to the flow of appearances? Explanation calls for a picture or story about reality beyond mere mathematical description. For this reason, continuity in space and time have been essential to causal explanation in physics. Intelligible pictures of how the world works are based on metaphors of everyday experience in which continuity plays a key role. What troubled Newton's contemporaries (and Newton himself), about gravity's apparent action at a distance, was the instantaneous leap across space and time-- the discontinuity that seemed to defy a causal chain. Various theories of gravity waves and gravity particles have attempted to bridge the gap. What is troubling about implications of the quantum theory is a similar causal discontinuity.
Modelling the world in terms of causes is also, of course, an everyday experience. There are stories we tell ourselves, and inner pictures we consult, to explain the motivations and actions of people. One has theories about how the world works. We are not only interested in foreseeing how people will behave but also want their behavior to make sense that we can assimilate to our own motivations. In daily living, we tend to demand certainty without the benefit of the accumulated evidence required in science. Thus, we are led to rely on belief, whose function is less prediction than satisfaction of an inner need for sense and certainty.