Locke on power
There is a useful exposition by Michael Ayers :
The idea of power is formed as follows:
The mind . .. concluding from what it has so constantly observed
to have been, that the like changes will for the future be made in the
same things, by like agents, and by the like ways, considers in one
thing the possibility of having any of its simple ideas changed, and
in another the possibility of making that change; and so comes by
that idea which we call power. Thus we say, fire has a power to melt
gold . . . and gold has a power to be melted. ... In which, and the
like cases, the power we consider is in reference to the change of
perceivable ideas (Essay 2.21.1).
What is Locke saying, in this and in surrounding passages? As the last
sentence quoted reminds us, he is concerned to show that the content of
our idea of power - and of our ideas of particular powers -extends no further
than we can observe. And yet the idea has reference to what we do not
observe, a postulated something in agent and patient, lying behind the
observed relationship between them. The idea of power is quite unlike the
idea of yellow, in that, first, it is not acquired in a special experience and,
secondly, it is used to refer to something beyond experience; but the corollary
is that it has no positive content by itself, since its positive content in any
particular case is supplied by the observable effect. It seems to me that
Locke sees it as operating in everyday thought as a sort of dummy concept,
faute de mieux.
In other words, the function of the idea springs from the rationality of
explaining observed interactions by underlying, unobserved properties of
the things interacting. But from observation we know these properties
only as whatever properties have such and such an effect in such and such
conditions. In Russellian terms, our knowledge is knowledge by description.
Thus it is entirely natural that, when he is assuming the atomic hypothesis,
Locke should sometimes identify the power with the actual primary qualities
supposed responsible for the effect: ". .. what is sweet, blue, or warm in
idea, is but the certain bulk, figure, and motion of the insensible parts in
the bodies themselves which we call so" (2.8.15).
It is not the observation of mere change, but of repeated, regular change,
that is said to give us the idea of power. The idea is acquired or suggested
to the mind by contact with things that always or nearly always behave
observably in a given way in given observable circumstances. It is because
we regularly observe the melting of gold whenever it comes into contact with
fire that we cannot reasonably avoid thinking in terms of continuously
existing but intermittently operative properties in each, which are independent of this relationship and reaction. Yet we come to see that the idea has
universal application, so that "whatever change is observed, the mind must
collect a power somewhere, able to make that change, as well as a possibility
in the thing itself to receive it" (2.21.4). Power becomes abstractly definable
as "the source from whence all action proceeds" (2.22.11)-not just observably regular action. (M. Ayers, 'The Ideas of Power and Substance in Locke's Philosophy', The Philosophical Quarterly (1950-), Vol. 25, No. 98 (Jan., 1975), pp. 1-27: 4-5.)
In fine, Locke regards power a necessitating factor lying behind observed relationships, inaccessible directly to experience, but responsible for, among other things, causal relations. Fire melts gold; we cannot observe what makes the gold melt (what would it be like to observe gold being made to melt but melt it does), and we assume a power at work producing and necessitating this effect.
Hume on power
Hume will have none of this; and in this respect is the more authentic empiricist. Here is what he says in the Treatise :
The idea of necessity arises from some impression. There is no
impression convey'd by our senses, which can give rise to that idea. It
must, therefore, be deriv'd from some internal impression or impression of reflexion. There is no internal impression, which has any
relation to the present business, but that propensity, which custom
produces, to pass from an object to the idea of its usual attendant.
This therefore is the essence of necessity. ( David Hume, A Treatise of Human Nature, L. A. Selby-Bigge and P. H.
Nidditch (eds) (Oxford: Clarendon Press, 1978), 165.
This connexion, therefore, which we feel in the mind, this customary
transition of the imagination from one object to its usual attendant, is
the sentiment or impression from which we form the idea of power or
necessary connexion. (David Hume, A Treatise of Human Nature, L. A. Selby-Bigge and P. H.
Nidditch (eds) (Oxford: Clarendon Press, 1978), 75.)
Hume agrees with Locke that power cannot be observed but he draws a different conclusion. While Locke conceives power as something we cannot observe but which is at work behind observation, Hume dismisses power from his philosophy. Since we cannot observe it, and all ideas derive from observation ('impressions'), we cannot make any justified use of the idea of power. It cannot explain anything. Rather, itself needs to be explained. Hume's explanation is that when one observation (impression) is regularly associated with another - there is a 'constant conjunction' between them - the ease with which the mind passes from one impression to the other creates an imaginary connection between them. We suppose that fire causes gold to melt, that fire has the power to melt gold. Nothing like this is true in Hume's view. Gold so constantly melts when exposed to fire that we suppose fire to have the power to melt gold. But all that experience yields is a regularity between gold's being exposed to fire and gold's melting. We observe no power at work here, readily as we may suppose that we do.