 This independent being is “self-existent, that is, necessarily existing” (D 12, W 2.527), a conclusion he also reaches by arguing that space and time cannot be conceived not to exist and they are obviously not self-existent, so the substance on which they depend, God, must exist necessarily as well (D 13, W 2.527–528). (Clarke was criticized for assuming, rather than arguing, that space and time are attributes of God.) It seems that for Clarke a “necessary being” is a being whose non-existence is impossible (either because it is an independent being or is necessarily dependent on an independent being), and a “self-existent being” is a being whose non-existence is impossible because the necessity of its existence is to be found in its own nature. Once these two are distinguished, however, Clarke is open to the criticism that he cannot rule out the existence of two self-existent beings (as he attempts in the seventh proposition) because there may be two beings who are self-existent even though only one self-existent being is required to explain the existence of the world (Law 1758, 21). Anthony Atkey (1725, 3–14) provided a variation on this objection in correspondence. He alleges that Clarke illegitimately moves from the existence of at least one “necessary being” to the existence of no more than one “self-existent being.” Clarke concludes that there exists only one self-existent being, but he has at best shown that we cannot have the idea of two self-existent beings. Atkey’s objection is about the relationship of the conceivable to the possible. Clarke’s response (Atkey 1725, 17–19) is that we have clear ideas in this case, so our ideas can guide us in the nature of things (conceivability entails possibility in the case of clear ideas), but he does not give any reason why we should think our ideas are clear in this case. This problem is exacerbated by his denial that we have adequate ideas of the essences of substances, including God. If we do not have adequate ideas of any substances (including the divine being) then how could we have a clear idea of God? Roger North had previously raised a similar worry about Clarke needing to show “natural things accord with our ideas,” but Clarke’s response has not survived (NC 133).