To the two attributes correspond two classes of modes. The most important modifications of extension are rest and motion. Among the modes of thought are understanding and will. These belong in the sphere of determinate and transitory being and do not hold of the _natura naturans_: God is exalted above all modality, above will and understanding, as above motion and rest. We must not assert of the _natura naturata_ (the world as the sum of all modes), as of the _natura naturans_, that its essence involves existence (I. _prop_. 24): we can conceive finite things as non-existent, as well as existent (_Epist_. 29). This constitutes their "contingency," which must by no means be interpreted as lawlessness. On the contrary, all that takes place in the world is most rigorously determined; every individual, finite, determinate thing and event is determined to its existence and action by another similarly finite and determinate thing or event, and this cause is, in turn, determined in its existence and action by a further finite mode, and so on to infinity (I. _prop_. 28). Because of this endlessness in the series there is no first or ultimate cause in the phenomenal world; all finite causes are second causes; the primary cause lies within the sphere of the infinite and is God himself. The modes are all subject to the constraint of an unbroken and endless nexus of efficient causes, which leaves room neither for chance, nor choice, nor ends. Nothing can be or happen otherwise than as it is and happens (I. _prop_. 29, 33).