In section ix of his A Treatise of Human Naure of 1740, David Hume rejected the doctrine of Passive Obedience [to royal authority], describing it as an “absurdity.” Two years later, in his Of Passive Obedience, he again rejected the doctrine of obedience and submission to sovereigns. Appealing to the maxim salus populi suprema lex esto, he argued that resistance to tyranny is “lawful or commendable,” but only when it is justified by necessity. This adds a new layer of meaning to the maxim insofar as it inverts the relation that until the eighteenth century held sway. That is, whereas until this point Necessity justified sovereign power, now Necessity is invoked in justification of its opposition. Just what Hume understands Necessity to mean, however, requires attention.
In his Of Liberty and Necessity, Hume defines ‘the doctrine of necessity’ as causality, determinism, and inevitability, describing its opposite as ‘chance’ – thereby equating the doctrine of necessity with Ananke. Liberty, he apprises us, is opposed not so much by Necessity as by something different: constraint. Liberty is the power to act or not act in relation to constraint. Placed in relation to necessity, liberty only appears to be liberty but is in fact closer to chance.
To the extent that this is the case, Hume’s invocation of Necessity as a justification of resistance requires some analysis. For, while Liberty has this dual meaning vis-à-vis Necessity, would this not imply that necessity is also related to something akin to constraint? After all, what is necessary is a constraint to some degree. At any event, what is interesting to note apropos our investigation of Salus is that Hume not only draws an equivalence between the doctrine of necessity and the inevitability of Ananke, but insofar as he defines Liberty as “a power of acting or not acting, according to the determinations of the will,” he equates liberty with health. This is so insofar as liberty, the power of acting or not – or, as we have seen earlier, liberating power – is opposed to the dominating power of necessity and fate. So, just as much as Ananke is opposed to Asclepius, Necessity/constraint is opposed to liberty - furthermore, liberty is constitutive of health.
Like Liberating Power, Liberty, this power of acting, according to Hume, is subject to the will, to a focusing of one’s energies. Insofar as it is closely related to the conatus and the world-spirit of the Stoics the will is also intimately associated with Salus. For example, Thomas Hobbes, who writes of a “will to survive,” in many respects employs the terms will and conatus interchangeably. Though coming from a different direction than Hobbes, Spinoza, as well, will replace the term ‘will’ with conatus. In the works of Arthur Schopenhauer, who writes of a Wille zum Leben, or “will to live”, we again see the association with a self-preservation instinct, which is also the domain of the conatus, coupled with that of the will. And, although he fails to distinguish the liberating from the dominating aspects of power, we again confront the intimate association of the will with the conatus in Nietzsche’s Wille zur Macht, or “will to power”, a conceptualization that would find echoes in, among other things, the ‘will to pleasure’, or pleasure principle, of Sigmund Freud. That conative energy, that radiance that is the healing power of salus is, if not indistinct from ‘the will’, at least that which the will focuses - all of which is consistent with the insight of Hegel that "the essence of spirit [conatus] is freedom." (Philosophy of History, §21)
 David Hume, Of Passive Obedience, 1742.
 Ibid. II.XIII.3
 David Hume, Of Liberty and Necessity, Part I
 Schmitter, Amy M. (2006), "Hobbes on the Emotions", Stanford Encyclopedia of Philosophy, http://plato.stanford.edu/entries/emotions-17th18th/LD3Hobbes.html, retrieved 2006-03-04
 Allison, Henry E. (1975), Benedict de Spinoza, San Diego: Twayne Publishers,