Aseity (from Latin a "from" and se "self", plus -ity) refers to the property by which a being exists in and of itself, from itself, or exists as so-and-such of and from itself. The word is often used to refer to the Christian belief that God contains within himself the cause of himself, is the first cause, or rather is simply uncaused, though many Jewish and Muslim theologians have also believed God to be independent in this way. Notions of aseity as the highest principle go back at least to Plato and have been in wide circulation since Augustine, though the use of the word 'aseity' began only in the Middle Ages.
Aseity has two aspects, one positive and one negative: absolute independence and self-existence. In its "negative" meaning, which emerged first in the history of thought, it affirms that God is uncaused, depending on no other being for the source of His existence. In its "positive" meaning, it affirms that God is completely self-sufficient, having within Himself the sufficient reason for His own existence. The first concept derives from "the God of philosophers", while the second one derives from "the living God of Revelation" (I am who I am: Exodus 3:14).
As a part of this belief God is said to be incapable of changing (see Hebrews 13:8) Changing implies development. Since God was, and is, and is to be the Absolute Perfection, there is no further need to change: he is αὐτουσία (unchanged: Gregory of Nyssa), actus purus and ipsum esse subsistens (Thomas Aquinas).
Many (St. Thomas, for instance) have also thought that aseity implies divine simplicity: that God has no parts of any kind (whether spatial, temporal, or abstract), since complexes depend on their individual parts, with none of which they are identical. Classical theists have often drawn a further implication: that God is without emotion or is "impassible": because, it is said, emotion implies standing as patient (pass-) to some agent – i.e., dependence. This is so because, although God has created everything, He is not in dependence on His creation.
Whether or not this being should be described as God turns on whether the label 'Creator' is a rigid designator of God. Given that most theists understand all that is not God to be brought about by God, and that many (for example, St. Aquinas) argue from the non-aseity of the universe to the existence of God, this problem is somewhat theoretical. There is also a possible threat to divine aseity by the existence of abstract objects. John 1:3 states that All things came into being through him, and without him not one thing came into being. The aorist tense implies that everything that exists (other than God) came into being at some time in the past. This verse carries the weighty metaphysical implication that there are no eternal entities apart from God, eternal either in the sense of existing atemporally or of existing sempiternally. Rather everything that exists, with the exception of God Himself, is the product of temporal becoming.
Aseity has also been criticized as being logically incompatible with the concept of God as a being or of God as existing. Furthermore, it can be argued that for the notion of aseity not to be logically circular or inconsistent, the supposed entity to which it applies would have to be identified with its properties, instead of instantiating, exemplifying or having its properties, and would therefore be a nonsentient force or potential of indeterminate vitality (see Monad). This, however, seems to contradict the notion that God is a person or a causal agent, for what person or agent can also be a property (or complex of properties)?