A Comment on the Letter by C. R. Galley, Phys. Rev. Lett. 110, 174301 (2013). 02.30.Jr Dissipative systems are ubiquitous in physics but their description in general mathematical terms is still a debated issue. Galley [1] proposed a general approach to the motion of a discrete dissipative system based on a modified Hamilton principle, which remedies the timereversibility of the Lagrange equations, which express the stationarity of the classical action. The major conceptual drift for developing this method was the supposed inability of the classical Rayleigh equations to account for resistive forces more general than linear functions in the velocities. It is my intention to show that a (slight) extension of Rayleigh classical formalism is able to encompass general dissipative potentials, also amenable to a variational formulation.The notation employed here for a discrete dynamical system is standard. The generalized coordinates form the vector q = (q 1 , . . . , q m ) ∈ R m ; correspondingly, q = (q 1 , . . . ,q m ) is the vector of generalized velocities. T (q,q) is the kinetic energy, taken to be a quadratic positive definite form inq; V (q) is the potential energy of all conservative generalized forces Q, so that Q = − ∂V ∂q , and L(q,q) := T − V is the Lagrangian of the system, which enjoys the usual smoothness assumptions. Letting H(q,q) := T + V be the total energy, it is a plain consequence of T being quadratic inq that (see also [2, p. 125]) H + W = 0, where(1) is the total mechanical power expended by active (Q) and inertial ( δT δq ) forces. Were these the only forces at work, by the Lagrange equations, δL δq would vanish identically along any motion, and so would W , implying that the total energy is conserved.We assume that nonconservative generalized forces can be expressed as − ∂R ∂q , where R(q,q) is a dissipation potential, not necessarily quadratic inq. Generalized Lagrange-Rayleigh equations can then be written in the traditional textbook form, δL δq − ∂R ∂q = 0, which combined with (1) transform the balance of energy intȯwhere D(q,q) is to be interpreted as the dissipation in the system. Here D is the only constitutive function for the nonconservative forces: it is positive semidefinite inq, but not necessarily quadratic. The dissipation potential R is determined in terms of D as a solution to the partial differential equation in (2) 2 . By applying the method of characteristics to this equation [3, Ch. II], we easily arrive at the following explicit representation for R (to within an arbitrary constant):A number of consequences can be drawn from (3). First, if D is a homogeneous function of degree n, then R = 1 n D, which for n = 2 reproduces the classical relationship between Rayleigh potential and dissipation. Second, if more generallyIn particular, letting D = A(q)|q| 3 , with A(q) ≧ 0, we recover the quadratic dependence on the velocity typical of the resistive drag opposed to bodies by air flows at high Reynolds numbers [4, Sec. 5.11], an example similarly encompassed by the method pr...