In thus opposing form and force to each other, Ruskin is actually referring to two kinds of forces. There exist those forces which resemble the potter's foot in producing mere numerically regulated movements (so that this part of the potter's activity can be replaced by a power-machine), and others, which like the potter's hand, strive for a certain end and so in the process create definite forms. Ruskin goes a step further still in The Queen of the Air, where he speaks of selective order as a mark of the spirit: