Has the Second Philosopher been too sanguine in resolutely dismissing scepticism? Here's a thought that might give her pause.
Hume starts off the Treatise as a cheerful naturalist. OK, he's not quite a naturalist of the Second Philosopher's stripe. She "won't follow Hume in placing the science of man (what she thinks of as psychology, sociology, linguistics, etc.) at the foundation of Natural Philosophy (what she thinks of as physics, chemistry, etc.) -- she tends to regard these various natural sciences as complementary parts of one big puzzle": but still she recognizes this Hume as a forerunner. Yet by the end of Book I, Hume's naturalistic starting point has seemingly driven him to a despairing skepticism about the senses. Is there something in his naturalistic stance that contains the seeds of its own undoing? Does this apply to naturalism more generally?
Some of course have argued so. Latterly, for example, Plantinga has argued that the contemporary naturalist will have to regard our cognitive mechanisms as quick-and-dirty adaptive devices which have evolved to get us successfully feeding, fighting, fleeing, and reproducing. And being success-conducive is one thing, truth-conductive is something else. The naturalist, by her own lights, has no good reason to suppose his deep-rooted cognitive habits are better than merely useful, and so should be sceptical about her own scientific endeavours, for scientific reasons.
I'll not discuss this (rather dismal) argument here, and Maddy doesn't either. But she does worry about how things went amiss, as she supposes, for Great Uncle David.
In the most general terms, his line of thought is familiar. We must distinguish between those perceptions and objects "which preserve a continued existence and identity" whether perceived or not. For arguments from the variability of our perceptions are supposed to show that "our sensible perceptions are not possessed of any [external] existence" (for, presenting inconsistent appearances, they can't all have "external existence", and since like effects have like causes, none of them do). The trouble then is, as Hume puts it in the Abstract, "the belief [in external objects], which attends experience" can't be defended by reason, so we are left with "nothing but a peculiar sentiment, or lively conception produced by habit. ... Philosophy wou'd render us entirely Pyrrhonian, were not nature too strong for it."
But what is Hume's conclusion here? That not all our beliefs are based on reason (we are not God-like enquirers, with rational insight into the nature of the world); many of our basic beliefs about the world around us are just produced in us by natural mechanisms operating at the same level as with other animals. So if 'knowledge' is conceived of as requiring reasoned beliefs, then we don't have much 'knowledge'. The naturalist can cheerfully agree with that, however. She doesn't share that concept of knowledge, and so she won't regard Hume's conclusion as sceptical in any worrying sense (for she's long since taken aboard the lesson that reasonable beliefs need not be reasoned beliefs).
Can we get something more alarming out of Hume, however? The Second Philosopher thinks not, or at least easily. Certainly, if some rather plausible-looking naturalist assumptions do lead to scepticism in the worrying sense, the sensible naturalist's first reaction will be to look around to see which dodgy planks on Neurath's boat are revealed on closer inspection to be letting in water, and do some running repairs if she can. For example, Hume writes
’Tis this principle [in effect, our ordinary cognitive norms], which makes us reason from causes and effects; and ’tis the same principle, which convinces us of the continued existence of external objects, when absent from the senses. But though these two operations be equally natural and necessary in the human mind, yet in some circumstances they are directly contrary, nor is it possible for us to reason justly and regularly from causes and effects, and at the same time believe the continued existence of matter.But now why suppose those conflicting principles really are both 'necessary', and then embrace a worrying scepticism (if that's what they lead to in Hume, contrary to the suggestion above)? As Maddy puts it:
[The Second Philosopher's] spirit of open inquiry suggests a different course [from Hume's]: if our cognitive norms lead to a contradiction, shouldn’t we re-examine our cognitive norms? Why should we assume it is impossible to amend or correct them?Which seems evidently right. Though Maddy adds
... Regarded second-philosophically, the despairing Hume hasn’t fully lived up to his guiding naturalistic impulse.And that's arguably too flat-footed about Hume. As hinted above I'd say that what the real Hume was giving up on was our supposed capacity for a God-like 'knowledge' as imagined by some of his predecessors (see Edward Craig's The Mind of God and the Works of Man for a lot more on this). And there's nothing despairing about rejecting that: it is exactly what a persistent naturalist needs to do.
Be that as it may, however, as a claim about Hume. And anyway, there are more twists and turns in Maddy's discussion as she confronts more than one Hume found by different interpreters. But she ends up in the same place each time: at least in Hume's writings, "[C]ommon sense and its scientific refinements have not been convicted of undercutting the reasonableness of their own methods."
That looks right. Though, advocating a sublunary naturalistic line on the reasonable, Hume could probably have agreed.