I'm studying section 82 of the Landau & Lifshitz Field Theory vol.2 In this page it's written that the relative position of test bodies can't remain unchanged during time. And ok with this. But then on the footnote they write
Strictly speaking, the number of particles should be greater than four. Since we can construct a tetrahedron from any six line segments, we can always, by a suitable definition of the reference system, make a system of four particles from an invariant tetrahedron. A fortiori, we can fix the particles relative to one another in systems of three or two particles.
What does this mean?