In Reilly's formulation, the attractiveness of the retail center becomes the analogy for size (mass) in the physical law of gravity.
The law presumes the geography of the area is flat without any rivers, roads or mountains to alter a consumer's decision of where to travel to buy goods.
In analogy with Newton's law of gravitation, the point of indifference is the point at which the "attractiveness" of the two retail centres (postulated to be proportional to their size and inversely proportional to the square of the distance to them) is equal: Where
As the size of A becomes very large with respect to B, d tends to D, meaning the customer will always prefer the larger centre unless they're very close to the smaller one.
Young in 1924 described a formula for migration that was based on the physical law of gravity, and H.C. Carey had included a description of the tendency of humans to "gravitate" together in an 1858 summary of social science theory.