How can the metre be define by the speed of light that was measured based on the old system of measurement?

[Hashirama_Senju]

To find the speed of light they had to use the measurement of the 'metre' that was not defined by the speed of light, so how can they define the new concept of the 'metre' based on the speed of light if the old 'metre' was measure with the old concept of the metre?

[swansont]

Some units have to be defined, rather than measured.

[Strange]

To add to that, we just switched from "an arbitrary definition of the metre which was used to measure the speed of light" to "an arbitrary definition of the speed of light which is now used to measure the metre".