Inspired by the philosophy of Herbert Spencer, Bergson sought to improve upon its inadequacies, which he believed to be Spencer's lack of comprehension when it came to the ideas of the mechanics of his time. This led Bergson to the conclusion that time eluded mathematics and science.
Bergson became aware that since time was mobile, the moment one attempted to measure a moment, it would be gone. What one measures is an immobile, complete line, whereas time is mobile and always incomplete. When one says that something happens at time T, all one means is that one will have counted a number T of simultaneities. For the individual, time could speed up or slow down, yet for science, the T of simultaneities would remain the same. Hence to explore the real time which science ignores, Bergson decided to explore the inner life of man, which is a kind of Duration.
For Bergson, this Duration is neither a unity nor a multiplicity. Because the Duration is, in fact, ineffable, the way toward understanding it can only be shown indirectly through images. These can never reveal a complete picture of the Duration, for it can only ever be grasped through a simple intuition of the imagination.
Thus, in An Introduction to Metaphysics, Bergson presents three images of the Duration to acquaint the reader with the idea. The first is that of two spools, one unrolling to represent the continuous flow of ageing as one feels oneself moving toward the end of their life span, the other a thread rolling into a ball to represent the continuous growth of memory as a person's past follows them. Indeed, for Bergson, consciousness equals memory. No two moments are identical, for the one will always contain the memory the other has left it. Therefore, a person can only experience two identical moments if they have no memory, but, Bergson says, that person's consciousness would be in a constant state of death and rebirth, which he identifies with unconsciousness.
The image of two spools is imperfect, however, as it involves the image of homogeneous and therefore commensurable thread, whereas, according to Bergson, no two moments can be the same, and hence the Duration is heterogeneous. Bergson then presents the image of a spectrum of a thousand gradually changing shades with a line of feeling running through them, being both effected by and keeping each of the shades it passes through. Yet even this image is inaccurate and incomplete, for it represents the Duration as a fixed and complete spectrum with all the shades spatially juxtaposed to one another, whereas there is no juxtaposition within the Duration, which is in reality incomplete and continuously growing, with the states not beginning or ending, but intermingling with one another.
Bergson’s final and most accurate image of Duration is that of an elastic band contracted to a point and then drawn out indefinitely to create a line which will progressively grow longer and longer. This image represents how Bergson identifies the Duration: an indivisible mobility.
Instead, let us imagine an infinitely small piece of elastic, contracted, if that were possible, to a mathematical point. Let us draw it out gradually in such a way as to bring out of the point a line which will grow progressively longer. Let us fix our attention not on the line as line, but on the action which traces it. Let us consider that this action, in spite of its duration, is indivisible if one supposes that it goes on without stopping; that, if we intercalate a stop in it, we make two actions of it instead of one and that each of these actions will then be the indivisible of which we speak; that it is not the moving act itself which is never indivisible, but the motionless line it lays down beneath it like a track in space. Let us take our mind off the space subtending the movement and concentrate solely on the movement itself, on the act of tension or extension, in short, on pure mobility. This time we shall have a more exact image of our development in duration.|20px|20px|Henri Bergson|The Creative Mind: An Introduction to Metaphysics, pages 164 to 165.
Even this image is incomplete, because the wealth of colouring is forgotten when it is invoked. But as the three images illustrate, it can be stated that the Duration is qualitative, unextended, a multiplicity yet a unity, mobile and continuously interpenetrating itself. Yet these concepts put side-by-side can never represent or give us the Duration itself, because it is mobile and always new, whereas concepts are always fixed, spatial and immobile. Thus if one ever wishes to grasp the Duration, they must make the effort to reverse the habitual modes of thought and place themselves within the Duration itself by using intuition.
Duration is first introduced by Bergson in his essay Time and Free Will: An Essay on the Immediate Data of Consciousness. It is used as a defence of freewill in a response to Immanuel Kant, who believed freewill was only possible outside of time and space, but since man cannot transcend time and space, must be accepted for pragmatic purposes alone.
Bergson’s response to Kant is that freewill is only possible within Duration, within which time resides as it really exists. The problem of freewill is not really a problem at all for Bergson, but merely a common confusion among philosophers caused by the immobile time of science being mistaken for the Duration.
To understand this, one must realize while space can be measured, the Duration cannot be. Thus to measure the Duration, it must be translated into the immobile, spatial time of science through a translation of the unextended into the extended. Although an essential practicality of both science and everyday life, it is through this very translation, mistaken for the Duration, that the problem of freewill arises. Since space is a homogeneous, quantitative multiplicity, the Duration becomes juxtaposed and converted into a succession of distinct parts, one coming after the other and therefore caused by one another.
In reality, however, the Duration is unextended. Nothing within it is juxtaposed and therefore nothing within the Duration can be the cause of anything else within the Duration. Hence determinism, the belief everything is determined by a prior cause, is an impossibility. To make the problem of freewill disappear, Bergson affirms we must accept time as it really is, which can only be experienced through placing oneself within the Duration, where freedom can finally be identified and experienced as pure mobility.
Zeno of Elea was a student of Parmenides, who believed reality was an uncreated and indestructible immobile whole. In defence of his master’s philosophy, Zeno formulated four paradoxes which present mobility as an impossibility. One of these, the Dichotomy paradox, will be used to demonstrate Bergson's riposte to Zeno's paradoxes, in a defence of both mobility and time.
Zeno’s basic argument is essentially that we can never move past a single point because each point is infinitely divisible and it is impossible to cross an infinite space. For example, say one wants to get from point A to point C. First one would have to move through point B. But to get from A to B, one would have to move through some halfway point between A and B, yet to get to this halfway point, one would have to pass through a point that is between A and the halfway point, and so on and so on ad infinitum. Since it is impossible to cross an infinite space mobility is impossible.
Bergson's response is to state that mobility is indivisible. The problem only arises when mobility and time, that is, Duration, are mistaken for the spatial line that underlies them. Bergson maintains that philosophers often make this mistake because the language they use is constructed by common sense in spatial jargon. Thus time and mobility are treated as things, not progresses, for practical measurement and, while an indivisible whole as they occur, are treated retrospectively as their spatial trajectory, which can be divided ad infinitum by a simple act of the mind.