 Here Stevenson seems to appeal to motivational internalism, or to the idea that, roughly, a moral judgment is necessarily accompanied with having some motivation to act accordingly (see Section 3.2 of the entry on  moral motivation),  or with a favorable attitude toward acting in the relevant way. Indeed, elsewhere Stevenson notes that “‘goodness’ must have, so to speak, a magnetism”: a person who “recognizes X to be good must ipso facto acquire a stronger tendency to act in its favour” (1937, 16).