Hume uses the example of the sun rising: though the sun has (so far) always risen in the east, there is no logical reason to assume that it will _necessarily_ do the same tomorrow tomorrow. Though we can reasonably plan for this as the most likely scenario based on available evidence, it is not _necessarily_ going to happen.

There is, likewise, no logically necessary reason to assume that that causal process of metabolism will continue to work tomorrow as it always has in the past, though it is indeed reasonable to prepare for that outcome as the most likely outcome.