An enquiry concerning the principles of natural knowledge

by Alfred North Whitehead

-time separation: and (3) if the event-particles be co-momental,

measures the 'proper' space separation and there is no 'proper' time separation; and if the particles be sequent,

measures the 'proper' time separation and there is no 'proper' space separation.

In the framing of physical laws it is essential to consider what measure of separation is relevant. It is to be noted that there may be time-systems

(other than

) of special relevance to the phenomena in question. It is not at all obvious that invariance of form in respect to all time-systems is a requisite in the complete expression of such laws; namely, the demand for relativistic equations is only of limited applicability.