Space-time symmetries and the principle of relativity alone suffice to obtain Lorentz-like coordinate transformations, in which a free parameter, k, plays the part of $c^{-2}$ in special relativity. Several authors have concluded that special relativity does not need the postulate of the constancy of the speed of light (the “second postulate”). I oppose this claim and argue that the transformations have no physical content unless different observers can ensure that they use the same units. The relativity of lengths and durations makes this non-trivial. One usually assumes boostability to solve this problem. I analyze this concept and show it is also non-trivial and cannot be assumed in general. The second postulate offers a way around this, as well as an economical method to match both time and length units. Without it, additional postulates are necessary to ground the physical meaning of the transformations.

仅时空对称和相对论就足以获得类似于Lorentz的坐标变换，其中自由参数k扮演了$C^{-2}$在狭义相对论中。一些作者得出的结论是，狭义相对论不需要光速不变的假设（“第二假设”）。我反对这种说法，并认为，除非不同的观察者可以确保使用相同的单位，否则这些转换没有物理内容。长度和持续时间的相对性使这一点变得微不足道。人们通常以增强能力来解决这一问题。我分析了这个概念，并表明它也是不平凡的，不能被普遍假定。第二个假设提供了解决此问题的方法，并且提供了一种经济的方法来匹配时间和长度单位。没有它，就必须有附加的假设以使转换的物理意义成为基础。