It is hard not to think that Hume was deranged if he thought that, when you put the kettle on the fire and it boiled, the putting the kettle on was not the cause of the boiling effect.
But there’s a lot of daylight between (a) denying causality and (b1) denying distinct human knowledge of (b2) a necessary causal connection between objects that are (b3) not directly related to interior human states. To explain:
(b1) The teaching referenced is from a section called Of Knowledge and Probability, in a book called Of the Understanding in a larger treatise not on reality but On Human Nature. We’re clearly speaking about what can be known about causes. Rather than speaking about causes in every possible sense, he speaks about causes as they can arise in the relation of a subject to its object.
(b2) Hume’s critique of causality goes only as far as his critique of necessary connection. Stripped to essentials, the argument is:
Necessary connection cannot be known.
Causes can be known so far as necessary connection can be.
Since Hume’s idea of necessity is what is true at all times, and we never perceive something at all times, then we can’t perceive a necessary connection. Prima facie, denying necessity becomes as self-evident as denying we can see the future.
It’s interesting to use this idea to contrast the Ancient and Contemporary accounts of science. Until Newton, it was generally thought that if we could not know causes, we could not have science since science was nothing but the logical order of causes in some subject area. Working from Hume’s account of cause and our Contemporary account of science, the exact opposite is true: if we knew causes, we could not verify predictions about anything, and science is most of all about verifying predictions. “Knowing causes” means the future is already given and so could not be predicted and later verified. Future data could not confirm scientific laws any more than future unmarried men could confirm our definition of a bachelor.
(b3) This is my own theory of Humian causality, but it is the only way I can make sense of how he rules out the possibility of causes in the First Book of the Treatise while he appeals to causes on almost every page of the Second Book. We can understand the causal relations between our own passions, appetites, knowledge, and other interior states because – unlike when we look at billiard balls – we can understand these active powers from the inside. The mere use of a straw can’t tell us whether we are pulling the liquid up or whether we are creating the conditions under which the atmosphere will push it down, but the mere use of the straw does involve a known relation between the straw and our thirst.
This also helps us to see why Howse’s objection to Hume’s theory is too ambiguous: when we put the pot on the fire the fire is being used as an instrument to our intention, and so far as we take it in this way there is real causality between the flame and the pot. But so far as we abstract from this and try to speak about causality in natural things, then we can object that fire no more obviously heats than our use of a straw pulls the liquid up. Or perhaps fire heats like the gallows-executioner drops the condemned man or like ice cools water. So far as we are talking about these things as instrumental to our intention, we can abstract from these differences, but as soon as we try to figure out the cause in itself (is it adding a form? Taking one away? Removing an impediment to one?) we see that, at the very least, the problem is a good deal more complicated, demanding that we make guesses that are capable of being confirmed by future evidence.