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, 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.

Often, as a part of this belief God is said to be incapable of changing. Changing implies development. Since God was and is and is to be the Absolute Perfection, there is no further need to change. 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. A further implication often drawn among classical theists has been that God is without emotion or is "impassible" for, 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.