 If your maxim fails the third step, you have a “perfect” duty admitting “of no exception in favor of inclination” to refrain from acting on that maxim (G 4:421). If your maxim fails the fourth step, you have an “imperfect” duty requiring you to pursue a policy that can admit of such exceptions. If your maxim passes all four steps, only then is acting on it morally permissible. Following Hill (1971), we can understand the difference in duties as formal: Perfect duties come in the form “One must never (or always) φ to the fullest extent possible in C”, while imperfect duties, since they require us to adopt an end, at least require that “One must sometimes and to some extent φ in C.” So, for instance, Kant held that the maxim of committing suicide to avoid future unhappiness did not pass the third step, the contradiction in conception test. Hence, one is forbidden to act on the maxim of committing suicide to avoid unhappiness. By contrast, the maxim of refusing to assist others in pursuit of their projects passes the contradiction in conception test, but fails the contradiction in the will test at the fourth step. Hence, we have a duty to sometimes and to some extent aid and assist others.