This last equation (without G) is valid with F ′, m 1 ′, m 2 ′, and r ′ being the dimensionless ratio quantities corresponding to the standard quantities, written e.g. F ′ ≘ F or F ′ = F / F P, but not as a direct equality of quantities. This may seem to be "setting the constants c, G, etc., to 1" if the correspondence of the quantities is thought of as equality. For this reason, Planck or other natural units should be employed with care. Referring to " G = c = 1 ", Paul S. Wesson wrote that, "Mathematically it is an acceptable trick which saves labour. Physically it represents a loss of information and can lead to confusion."