‘LIBERTY, or FREEDOM,’ Hobbes states, ‘signifieth, properly, the absence of opposition; by opposition, I mean external impediments of motion …’ We naturally ask: what about internal impediments? And Hobbes answers, ‘… when the impediment of motion, is in the constitution of the thing itself, we use not to say, it wants the liberty; but the power to move; as when a stone lies still, or a man is fastened to his bed by sickness.’Footnote ¹ Whatever we say about the ‘internal impediments’ that prevent any movement by Hobbes' stationary stone, he doubtless is in agreement with ordinary English usage when it comes to the man bed-ridden by sickness; but that doesn't make his answer any less unconvincing philosophically. From the standpoint of what interests philosophy, our freedom – our range of options – is as truly curtailed by sickness as by shackles.

The same thing goes for psychological facts about us which stand in the way of our action. In a locked room, ignorance of the lock's combination will indeed deprive us of the freedom to depart. Inattentiveness and the force of habit as well as biased preconceptions can likewise play their part as freedom-curtailing impediments to action. (As for correct beliefs that prevent us from acting the way we otherwise would, they are impediments to such actions, no doubt, but they are not freedom-curtailing insofar as what they do is to convince us correctly that those actions would not really be in the interests of that which we most desire.)

However, what about desires which we have? Freedom/determinism compatibilists including Hobbes have in effect defended the conception of ‘freedom’, more or less, as: being in a position to attain what one desires. And contrary-tending desires certainly can stand in the way of that. Harry Frankfurt points out how unfree the unwilling addict is. Though conflicting desires can be seen as IMPEDIMENTS to each other's realization, Frankfurt identifies the freedom of the person concerned with the ability to realize the desires which the person predominantly prefers to have (or, if there is a still higher-order desire preference, he identifies the person with that).Footnote ²

What, though, if such an individual is ‘of two minds’, either undecided, or else sometimes preferring to possess, and to gratify, one lot of desires and sometimes another, contrary lot? Which lot of desires is it fulfilling to the person to gratify, and which lot of desires act as liberty-curtailing impediments? In default of a good theory of what is ‘essential’ in a human being's motivation, we shall have to fall back simply on whatever such an individual prefers most of the time; or else, simply shrug and concede that there the philosophical concept of liberty at least partly breaks down.

There are incompatibilist writers, on the other hand, who maintain that people are free only when motivationally conflicted.Footnote ³ This is without doubt due to the thought that only for those is there more than one way that they could act. But science-based psychological determinism need not deny that it is equally possible FOR a person to act in either of two ways solely because, on a given occasion, available evidence stops it from being equally possible THAT the person will act in either of those two ways. There is an ambiguity as between the epistemic and the achievability senses of ‘could’, ‘can’, ‘must’, ‘possible’, ‘impossible’, ‘necessary’ and ‘necessitate’ (and hence in the meaning of ‘determine’, that synonym of ‘necessitate’). In one sense ‘impossible’ marks the extreme limit of improbability, in the other sense the extreme limit of difficulty.

Attentive to the unanalyzed ‘possibility’ of acting differently from the way one does, Susan Wolf is incompatibilist enough to consider psychological determinism inconsistent with the moral freedom requisite for a judgement that someone has acted immorally; but she recognizes that from a moral point of view actions deterministically motivated by an unconflicted good will are paradigmatically praiseworthy.

Should we not, rather, say that, while such an individual in one sense cannot do otherwise, in another sense he perfectly well can? That is, there is no difficulty involved in doing otherwise here, but only unwillingness on his part. So that which it is perfectly ‘possible’ FOR him to do is something there is little ‘possibility’ THAT he will do.

A righteous or a wicked individual with no motivational conflict (and otherwise free) is surely unimpeded in doing whatever acts of commission or omission that individual does. But this gives rise here to an asymmetry different from the one Wolf claims.

Anything external sufficient to prevent (or hinder) you from doing an action counts as an impediment to your doing it whether or not you have any desire to do it. And the same thing goes for psychological or other internal factors apart from desires: they do not need to do any actual impeding in order to be impediments. Your ignorance of the lock's combination deprives you of the freedom to leave the room even if you have absolutely no desire to leave it.

On the other hand, when the desires composing an unconflicted will are sufficient to prevent someone from acting in one way rather than in another, those desires do not count as impediments to that way of acting. In such a case, where no impeding is involved, just this fact is enough to stop those desires from counting as impediments. Where your only desire is to do something and you successfully carry out that desire, if there are any impediments to acting in some other way, they will not include that desire. Whereas other causes that act to prevent you from doing something are indeed effective impediments to your doing it.