Hume’s Account of Liberty and Necessity

The Project

Hume presents his views on freedom in both A Treatise Human Nature (1739) (T hereafter) and An Enquiry Concerning Human Understanding (1748) (EU hereafter). In T, Hume presents a hard determinist view, where liberty seems incompatible with necessity. Hume mostly argues for his compatibilist thesis in EU.

By the time Hume wrote EU, he had two general aims: (1) reconciling liberty with necessity; (2) showing that morality requires necessity. We should understand this project in light of Hume’s empiricism and methodology. Hume wanted a ‘Newtonianism’ of the mind and to eschew metaphysical speculation (Harris, “Hume’s Reconciling Project” (2005), pp. 64-5).

Causation and Necessity: Reconciling Liberty with Necessity

For Hume, all ideas are copied from (or stem from) impressions [copy principle]. Without an impression of red, for example, we could not have the concept red. The will is just the internal impression we are conscious of when we give rise to a new motion of our body or new perception of our mind (T Thinking about the will inevitably leads to questions about liberty and necessity.

Necessity, Hume thinks, is never perceived between objects. What we do perceive are objects in constant conjunction. Necessity just is this constant conjunction of objects together with an (inductive) inference of the mind from one object to another. For Hume, one cannot define cause without the notion of necessity, so it makes little sense in distinguishing between necessary and unnecessary causes (EU, 8.25). In short, all causes are necessary.

Hume also thinks that this necessity extends to human behavior. In particular, the constant union between motives and actions has the same constancy as any other set of objects. It follows, according to Hume, that the influence of this constant union on the mind is the same as in all other natural operations: they determine us to infer from the existence of one to the existence of the other (T,

The central question we are concerned with here is how Hume reconciles liberty with necessity (and this will turn on Hume’s particular understanding of causation and necessity). One of Hume’s central contribution to the free-will debate is this peculiar conception of necessity. This conception of necessity is (presumably) perfectly compatible with liberty.

 Problems and Open Questions

  • How do we individuate events/objects in the right way?
  • There is a certain regularity in our impressions (the constant conjunction of objects), but how do we isolate or pick out objects that are appropriately related? This regularity seems to need to be, at least, temporally ordered.
  • to what extent is Hume really a compatibilist? Is Hume even a determinist?

Hume’s Sentimentalism: Morality requires necessity.

Paul Russell (2007, “Hume on Free Will”) identifies two key arguments operative in Hume’s writings regarding the connection between morality and necessity. The “liberty argument” begins by asserting that liberty consists in spontaneity (freedom from constraints) rather than indifference (freedom to act or refrain from acting).  Therefore the crucial difference between free and unfree actions consists in the type of underlying cause (rather than the presence/absence of a cause); free actions are caused by the agent’s willings, unfree actions by external circumstances.  The “necessity argument” seeks to prove that necessity, properly understood, consists of nothing more than constant conjunction between events along with the mind’s habit of inferring one event from the other.  In this diluted sense, necessity poses very little threat to our freedom.

In fact, true freedom seems to require a necessary connection between a person’s character and her actions.  This important result follows from Hume’s sentimentalism, which asserts that moral judgments amount to nothing more than expressions of approval and disapproval.  Only character traits, Hume claims, can be the proper objects of moral sentiment.  Furthermore, our attribution of responsibility to other agents indicates that we possess knowledge of (or perhaps even just beliefs about) their overall character.  However, the only way to access a person’s character is by drawing inferences from her actions.  Therefore, in order to guarantee that our understanding of a person’s character is genuine, and that our attributions of approval and disapproval are apt, we must antecedently assume a tight connection (indeed, a necessary connection) between her character and her actions.

Divine Determinism: The “Author of Sin”

One powerful objection against this Humean view of morality stems from theological considerations.  If voluntary actions admit of necessary connections in the same way that interactions among matter do, then we seem to be faced with the following dilemma: either all human action must be completely free of moral turpitude (for every action originates from God, the supremely good cause), or God must be unspeakably criminal and debased for giving rise to the moral turpitude that exists (EU 8.32).  Looming in the background of this dilemma is the assumption that responsibility for some action (and, in particular, responsibility for evil deeds) lies in the agent that was the ultimate causal source of that action, rather than some intermediary.

Hume’s response to this objection is not altogether satisfactory.  Pointing out that the reconciliation called for has “been found hitherto to exceed all power of philosophy,” he seems content to leave the issue a mystery, and restrict philosophy’s jurisdiction to “the examination of common life” (ibid.).  Perhaps this is just a savvy way for a closet-atheist to avoid wasting his time on theology without criticizing the discipline outright; or perhaps he even aims to undermine the legitimacy of theological reasoning by remaining reticent.  Nevertheless, there seems to be a strong response available to Hume.  He can claim that, insofar as God’s purview concerns the overall salvation of mankind, sporadic instances of moral turpitude can be outweighed by the good that they ultimately bring about.  This line of argument allows us to preserve our disapproval of a given person’s debased character, however, since each individual’s moral scope is obviously narrower than God’s.

It would be reasonable to assert at this juncture that this reply to the “author of sin” objection, though perhaps an adequate response when considered in isolation, would not cohere well with Hume’s skeptical critique of theology.  The point is apt, and to address it would require a lengthy discussion of Hume’s delicate views on religion.

Further Problems and Open Questions:

  • How to differentiate between internal and external causes?
  • Can actions be the object of approval and disapproval? (E.g., we needn’t know anything about a slave-owner’s character traits to know that her actions merit disapproval.)
  • Does this argument actually establish a necessary connection between character and action?  Or would a highly probable correlation suffice?



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