Contemporary Humeans, like Hume himself, claim that a clear characterisation of causal necessity requires more than the mere mention of such words as 'power' and 'productive connection'. In order to explain causal necessity within the framework of a phenomenal time-slice ontology, Humeans have developed highly sophisticated regularity theories of causation in terms of subjunctive and counterfactual conditionals and inductive laws. One main consequence of Hume's 'regular contiguous succession' formula for causation is that there are no a priori type or 'sort' restrictions on the phenomenal items associated together through the causal relation. Hume's account of the once-off perception of causal connections leaves the notion unexamined, though his account is heavily dependent upon it. In a word, Hume's arguments against an impression of necessary causal connection are unsatisfactory and inconclusive. Hume's view offers merely a description of regular relationships between certain mental and physical phenomena.