See also the related post on the *Center of vass*. Relativity has been addressed before, such as *here*.

Isaac Newton called mass “the quantity of matter”, which is still used sometimes, although Max Jammer points out how it has been criticized for centuries (see *Concepts of Mass in Classical and Modern Physics*, 1961). Other definitions arose in the 19th century. One is the ratio of force to acceleration, which assumes that force can be defined independently of mass.

Another approach is to define the equality of masses. For some such as Saint-Venant, “two bodies have equal masses if their velocity increments after impact are equal.” (ibid., p.91) For Ernst Mach equal masses “induce mutually equal and opposite accelerations.” (ibid., p.94)

Is there an independent definition of *vass*, the inverse of mass? One could modify these definitions of mass to define vass or equality of vasses:

Definition 1: *Vass* is the ratio of the *surge* to the *modulation* of a movement.

Definition 2: Two subjects have equal *vasses* if their *legerity* increments after impact are equal.

Definition 3: Two subjects have equal *vasses* if they induce mutually equal and opposite *modulations*.

In relativity theory, mass is dependent on velocity as follows:

*m* = *γ m*_{0},

with mass *m*, invariant mass *m*_{0}, velocity *v*, speed of light *c*, and *γ* = (1 – *v*²/*c*²)^{–1/2}.

It is easily verified that vass is dependent on legerity as follows:

*n* = *n*_{0} / *γ*,

with vass *n*, invariant vass *n*_{0}, legerity *u*, pace of light *ç*, and *γ* = (1 – *ç*²/*u*²)^{–1/2}.