An important element of Thomistic metaphysics is the role of existence and essence. Within creatures there is a distinction which must be made between existence and essence. However for God, his essence is his existence. This is important to note because many theological and philosophical errors have derived from a failure to recognize these two distinctions.
First, within creatures existence and essence cannot be convertible. Every material substance, (spiritual substances differ slightly) is the combination of a substantial form and matter. Hence, each individual material substance has an individuating principle, namely matter, and a universal quiddity which provides its nature, namely its substantial form. This is required for twofold reasons which are relevant for our discussion. First, a substantial form provides a material substance with its nature, providing its regular functions and its limitations. Second, a substantial form actualizes primary matter, yet secondary matter still remains which leaves open possibilities for the material substance to change. Hence, this hylomorphic understanding of material substances allows for the necessity of all things to change. Moreover, this change is only possible because the secondary matter which exists in the material substance, allowing for the potential of a substantial change. This can only occur if within material substances, both existence and essence are not convertible. If existence and essence were convertible within created material substances, then there would be no change possible within substances. In other words, we would be left with a complete monism, not unlike Spinoza. This is obviously in error because it cannot account for a plurality of beings or substantial change, nor ultimately the distinction between creator and created. Hence, from the above we can conclude that in material substances there is a real distinction between essence and existence.
Second, within God, his essence must be his existence. According to Thomas, God is that which necessarily is. Moreover, if that which is was different than his existence, than that essence would be in variance with his existence, as in a quality which is in variance with its substance. However this is impossible in God, because that would create a division with the Godhead, which is a logical impossibility as that which necessarily is, is also necessarily one. In other words, if God has an essence which was different than his existence, then there would be a division in God between what he is and that he is. Moreover, this division has several repercussions that lead to error. First, such a division would mean that God’s essence was dependent upon his existence, which would mean that within God was something dependent, not pure actuality. Second, such a division would mean that God’s essence was something superadded to his existence, and hence something would need to be posited outside of God which can then can add into God his essence, which is a clear absurdity. Third, if there was a division between God’s essence and existence, it would mean that there could be change within God. As I mentioned above, the reason that material substances can change is because there is a separation between our existence and essence, which can account for a potentiality to be something other than we are. However, if this was also the case with God, it would mean that God had a potentiality to be something other than he is, in which case he could also change who he is to be something else. This is obviously absurd, as it violates the principle that God is pure act, changeless and perfect. Finally and ultimately, if God’s essence and existence were different, we would logically have to posit a God above God. This is because if God had potentiality and thus not pure act, we would have to ask the question, where did God get his potentiality from, except from that which is pure act and which can have no distinctions? Hence, from the above, there can be no distinction between existence and essence in God.
