According to An Enquiry Concerning Human Understanding (Links to an external site.), Hume believes that what we should mean by “necessity” is that something seems to follow a regular correlation.
“[This] fire necessarily causes heat” just means that, in the past, we have always seen that things like this fire were accompanied by heat. Now, we usually believe such necessary correlations exist, and mean something more than (Hume’s definition of) “necessary.”
How do we typically define necessary? Are human actions an exception? Are they different than other kinds of actions?