We are determined by custom alone to suppose the future conformable to the past. When I see a billiard-ball moving towards another, my mind is immediately carry’d by habit to the usual effect, and anticipates my sight by conceiving the second ball in motion. There is nothing in these objects, abstractly considered, and independent of experience, which leads me to form any such conclusion: and even after I have had experience of many repeated effects of this kind, there is no argument, which determines me to suppose, that the effect will be conformable to past experience. The powers, by which bodies operate, are entirely unknown. We perceive only their sensible qualities: and what reason have we to think, that the same powers will always be conjoined with the same sensible qualities? ’
Tis not, therefore, reason, which is the guide of life, but custom. That alone determines the mind, in all instances, to suppose the future conformable to the past. However easy this step may seem, reason would never, to all eternity, be able to make it.--David Hume Abstract 15-16.
In the Treatise (1.3.6), the problem of induction is presented with incredible rapidity and without much obvious signposting that it may well be significant. I am pretty sure that the first time I read the Treatise I missed its significance. There the problem is presented in the context of (to use anachronism) the cognitive mechanism responsible for causal ascription. Hume suggests that the mechanism, association, is in the faculty of the imagination and relies on the calibrated effects of habit/custom.*
Terseness need not be a sign of insignificance in a subtle thinker. And, in fact, when in the Abstract, Hume decides to offer a summary even presentation of the main points of the Treatise the material of 1.3.6 gets pride of place and is articulated in a new manner. (There has been excellent work on this by Peter Millican from his dissertation onward, so I won't explain all the differences.)But in re-reading it, I was surprised that Hume would write in this context,"The powers, by which bodies operate, are entirely unknown. We perceive only their sensible qualities: and what reason have we to think, that the same powers will always be conjoined with the same sensible qualities?" Why focus on the hidden powers of bodies when responding to his (solution of) the problem of induction?
Before I get to that, I want to make two general observations. First, in reading the Treatise, one may think that the problem of induction is really an artifact of (Kant-inspired) reception of Hume, but not really a big deal in Hume's science of man or cognitive science. After all, throughout his scientific work (e.g., his political economy, his cognitive science, his history), Hume makes all kinds of causal ascription without worry about the problem. (This thought comes naturally to a 'naturalist' reader of Hume.) This line of thought can draw not only on the terseness of Hume's discussion in the Treatise. It can also point to Hume's (much more elaborate) account in the first Enquiry, where Hume emphasizes, in direct context, that his kind of theoretical philosophy (the skeptical) is largely impotent in changing anything in the real world (5.1-2)--even if the problem of induction is a genuine problem in speculative philosophy, it's harmless and can be screened off from our other (practical and ordinary) interests.
Second, even so, the passage quoted above from the Abstract follows immediately on Hume's statement of the problem of induction. That custom is a "guide" to life is not an insignificant result (see also EHU 5.6): it connects to Hume's political philosophy (where custom is a source of authority of institutions), and -- if custum guides our actions, where is there room for us to exercise our will? -- is it connects to Hume's treatment of free will (as Hume recognizes in EHU 8.11). So, Hume's response to the problem of induction shapes quite a bit of his practical philosophy. (There is more to be said about this.)
Okay, with that out of the way, let me return to the passage that puzzled me. Hume treats the occasion of the problem of induction to say something about the inaccessibility of hidden powers to us. To explain this we need to recognize that, in dialectical context, Hume is presupposing (i) an explanatory model familiar from Locke (and Spinoza, Aristotle, Boyle, etc.) in which hidden powers (insensible qualities) of an entity are causally responsible for the sensible qualities. These hidden powers may be (part of) the essence of the entity, but that need not be the case. So, Hume's implied targets accept something akin to (i).** That is not insignificant because (i) is the main explanatory workhorse/template of early modern science.
Now, if one accepts (i), then the problem of induction reveals two (related) problems: (a) one concerns the connection between sensible qualities, and (b) the other concerns the connection between hidden qualities and the visible qualities. So, if one accepts (i), one might have thought, for example, that routinely (if one wishes, exception-less) conjoined sensible qualities must always presuppose the same combination of hidden/invisible qualities that produce them. So, one could solve the problem associated with (a) by an appeal to hidden qualities. In thinking this one could, in fact, appeal to a same effect/same cause principle of the sort Newton articulates in the second rule of reasoning: "to the same natural effects we must, as far as possible, assign the same causes." To be sure, Newton's version of the rule recognizes exceptions and is clearly regulative ('as far as possible') and it does not rely on (i). Rather, in Newton the rule is a direct consequence of a parsimony principle ("admit no more causes of natural things than such as are both true and sufficient to explain their appearances").
But as Hume discerns, the explanatory model of (i) lacks resources to secure anything like 'same visible effect/same hidden cause.' That's because Hume thinks there is always a barrier in moving from sensible qualities to the structure of hidden qualities. (In the literature on Newton, also rejects (i), this is called the 'transduction problem' since an influential article by McGuire.) He makes this point fully explicit in the first Enquiry: "he has not, by all his experience, acquired any idea or knowledge of the secret power, by which the one object produces the other" (5.4).
This is why Hume's philosophy of science is so deflationary: all any science can show is (as Cassirer emphasizes) the relations that obtain among visible qualities. These relations can be grouped together and given names ("elasticity, gravity, cohesion of parts, communication of motion by impulse") and be treated as "causes" of the phenomena, but these names do not signify or explain features of hidden powers. In Hume's philosophy of science, science remains at the surface. Hume made the point explicit in a note added to the Treatise: "If the Newtonian philosophy be rightly understood it will be found to mean no more. A vacuum is asserted: That is, bodies are said to be plac’d after such a manner, as to receive bodies betwixt them, without impulsion or penetration. The real nature of this position of bodies is unknown. We are only acquainted with its effects on the senses, and its power of receiving body."
This barrier between visible and hidden qualities/powers is independent of the more fundamental problem of induction that even when one grasps, in some way, the structure of the hidden qualities ordinarily responsible for conjunction of sensible qualities, one cannot feel secure about their presence and role in any claim one makes about sensible qualities. For, a different set of hidden qualities may be responsible for any ordinarily conjoined set of sensible qualities. (Hume here discerns the outlines of the problem of multiple realization as a general metaphysical problem.) That, in addition, nature may change its course (and so change the configuration of hidden qualities that produce sensible qualities) means that the the main explanatory framework of early modern science (i) is without foundation.
*He concludes his discussion with a very puzzling sentence, "Thus tho’ causation be a philosophical relation, as implying contiguity, succession, and constant conjunction, yet ’tis only so far as it is a natural relation, and produces an union among our ideas, that we are able to reason upon it, or draw any inference from it." (1.3.6.16) About that more elsewhere.
**I think Hume himself never accepts (i), but that's disputed territory (in part connected to the New Hume debate).
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