 As we saw above, Hume also believed that causal statements entail counterfactuals, but the question is whether causal statements are definable in terms of counterfactuals. Here is an objection against such an attempt: Consider the following example. Because of severe frost during the night, ice covers the lake this morning. So, given the circumstances, if it had not been severe frost, there would not have been any ice on the lake. However, in these circumstances, the frost as the cause of the ice seems to be not only causally necessary for the ice, but the frost also seems to be causally sufficient for the ice cover. In other words, given the actual circumstances, it seems correct to say that the frost is causally sufficient as well as necessary for the ice. If we are going to represent causally sufficiency in terms of a counterfactual statement, we might then say that, given the actual circumstances, if no ice had covered the lake this morning, there would not have been a severe frost during the night. But if one accepts this objection, it shows that causal statements cannot be defined in terms of counterfactuals because such a definition does not give us the wanted asymmetry between cause and effect since each is counterfactually dependent on the other.