As a thought experiment, consider a rifle bullet, conceived of as an inertial projectile, fired at a target. Let the bullet itself be a source of measurement units: there is the length of the bullet and the rotation of the bullet. The extent of the motion of the bullet to the target could then in principle be measured as a number of bullet lengths and a number of rotations.

Length is the number of bullet lengths. Time is the number of bullet rotations. Thus length is essentially a linear measure and time is essentially a rotational measure. Length is generalized as the correspondence of the motion to a linear object, a rigid rod or ruler, which forms the basis of space. Time is generalized as the correspondence of the motion to a rotational object, a clock, which forms the basis of (abstract) time.

A rifle bullet provides a way to conceive of motion coordinates. Consider individually labeled rifle bullets continually fired from a common origin toward three orthogonal directions. The coordinates of a particular motion are then the labels on the coordinates that correspond to the motion. This means there are three pairs of coordinates: two for each bullet. These axes are the six degrees of freedom.

Motion conceived as a length function of time means that each for each rotational coordinate there corresponds its paired length coordinate. Motion conceived as a time function of length means that for each length coordinate there corresponds one paired rotation coordinate.

In conclusion, there are three dimensions of mobility. There are three dimensions for each measure of the extent of motion, which totals six dimensions. For ordinary purposes, the three dimensions of motion are sufficient, with space and time kept separate. But for science, which seeks a unified treatment, space and time should be united into six dimensions.

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