The contingency of any phenomenon or event must depend upon the nature of its cause. A contingent phenomenon or event is one which may be conceived of, as one that might not have been at all, or might have been different from what it is; but wherein lies the possibility that it might not have been at all, or might have been different from what it is? This possibility cannot lie in itself, for an effect can determine nothing in relation to its own existence. Neither can it lie in anything which is not its cause, for this can determine nothing in relation to its existence. The cause therefore which actually gives it existence, and existence under its particular form, can alone contain the possibility of its not having existed at all, or of its having existed under a different form. But what is the nature of such a cause? It is a cause which in determining a particular event, has at the very moment of doing so, the power of determining an opposite event. It is a cause not chained to any class of effects by its correlation to a certain class of objects--as fire, for example, is chained to combustion by its correlation to a certain class of objects which we thence call combustibles. It is a cause which must have this peculiarity in opposition to all other causes, that it forbears of itself to produce an effect which it may produce, and of any given number of effects alike within its power, it may take any one of them in opposition to all the others; and at the very moment it takes one effect, it has the power of taking any other. It is a cause contingent and not necessitated. The contingency of the event, therefore, arises from the contingency of the cause. Now every cause must be a necessary or not necessary cause. A necessary cause is one which cannot be conceived of as having power to act differently from its actual developements--fire must burn--gravitation must draw bodies towards the earth's centre. If there be any cause opposed to this, it can be only the contingent cause above defined, for there is no third conception. We must choose therefore between a universal and absolute necessity, and the existence of contingent causes. If we take necessity to be universal and absolute, then we must take all the consequences, likewise, as deduced in part II. There is no possible escape from this. As then all causes must be either necessary or contingent, we bring will under the idea of contingency, by regarding it as a contingent cause--"a power to do, or not to do,"[5]--or a faculty of determining "to do, or not to do something which we conceive to be in our power."[6]