Ethical motivation represents an important aspect of Kant's practical philosophy, one without which much of Kant's distinctive position would be lost. Not surprisingly, it is also one of those aspects of Kantianism to which Hegelian criticism directs its focus with predilection. Central to Kant's account of moral motivation is the distinction between acting merely in accordance with duty and acting from duty. When he introduces this distinction, in the Groundwork, Kant also points to the epistemic difficulties of properly drawing the distinction. A key concept here is, without any doubt, that of duty, and Kant begins with a preliminary definition: the notion of duty is a notion ‘which contains that of a good will though under certain subjective limitations and hindrances’ (G: 4: 397).
What this definition tells us is that, although beings which are only governed by practical reason without any admixture of inclinations and sensuous drives, that is, purely rational beings, will also have a good will, such beings do not have duties precisely because they lack the ‘subjective limitations and hindrances’ of sensuous motivating forces, such as desires, passions, habitual responses. If a person spontaneously and necessarily acts as duty requires, then it does not make sense to talk about an obligation for this person to act as duty requires. Such a person must be a purely rational person, since only she can always and necessarily act as (practical) reason requires. By contrast, beings with limitations and hindrances, like us, act spontaneously and necessarily as natural laws require and, hence, it does not make sense to talk about our obligation or duty to observe the laws of nature.
On Immanuel Kant’s Concept of Duty