The speed of light is a tautology. We define it via how many meters light travels in a second. And we define the meter by the same measure. It’s just the distance light travels in 1⁄299792458

c is a measurable constant, not come unit that is arbitrarily defined. Like Boltzmann’s Constant, or the ground state hyperfine transition frequency of the Cesium-133 atom… it just… Is.

Therefore, it is a useful tool to define units. You claim it is a tautology because we write it in units of meters per second, while the meter is defined based on c. This is easily disproven, as you can represent the speed of light in any unit of velocity. It is a fundamental constant, derivable through experiment without any unit a priori.

It’s not about the units i used. It’s about using something to define itself. The same problem happens when you use c to define empty space since empty space can define c.

Once you decide which units are used in maxwells equations then the electromagnetic permeabikity and permissivity pops out as a proportions of c.

Read more Feynman if you don’t believe me.

That may be, and I’ve been meaning to dig into my copy of the Lectures, but that’s moving the goalposts. You said that it was a tautology because it was defined by the meter, and the meter was defined on it. That statement is demonstrably false.