666 IX. ON EMPIRICAL AND VERBAL STRUCTURES
system, the world of our experience is represented by a fourfold order. We can order the events as to the right and to the left, forward and backward, up and down, and sooner and later. In our experience this fourfold order is completely united, and cannot be separated unless we deliberately choose to neglect some of these orderings.
Nor does it mean that all these'dimensions are 'identical'. We are accustomed, for instance, to consider the three dimensions of 'space' as 'identical', or at least equivalent. Is this true in life? Can we disregard, for instance, the structural difference between vertical and horizontal? If we did, quite probably, as Eddington remarks, we should come to an untimely end, and break our necks.
Obviously if we visualize our plenum, as made up of lines or particles, by necessity we visualize structurally a four-dimensional manifold. It should be noticed that a four-dimensional 'absolute void', or 'absolute nothingness', besides being non-sense, cannot be visualized at all, because it could have no structure.
We see that all metaphysical 'fourth dimensions' are not only non-sense, but usually indicate a pathological semantic disturbance. The intensity of such disturbances is often high, because it is entirely impossible for a sane person to deal with such meaningless noises. The victim is obsessed with attempts to do the impossible, - a semantically hopeless and painful task.
Such objectifications of terms are very dangerous and science should try, by proper emphasis, to eliminate them. Outside of science the term 'dimension' has no meaning and ought to be definitely abandoned in our speculations, for the sake of sanity.
The notion of 'time' as a 'fourth dimension' is by no means new. It appeared in a vague form centuries ago. The notion however was not formulated properly, and therefore was unworkable. Instead of helping science, it only hindered it.
Inspired by Einstein's work, the mathematician Minkowski, whose work had been mainly in the theory of numbers, began to work at the theory of manifolds of any number of dimensions. In 1908 he delivered his famous and semantically epoch-making address on Space and Time which fused geometry and physics structurally. In this address he insisted that the connection between 'space' and 'time' as given by the Lorentz-Einstein formulae is not accidental but exhibits that inner connection or structure to which we had not paid enough attention.2
In our experience, 'space' and 'time' can never be entirely separated, as already explained, and so Minkowski combined them into a higher entity which is called the 'Minkowski world'. In the world of experience the datum appears to be, not a place and a point of 'time', but the event or the world point - that is, a place at a definite date.
The graphic picture of a moving point is a world-line. Rectilinear uniform motion corresponds then to a straight world-line; accelerated motion, to one that is curved.