This consideration also gives us the key to that _contentio_ περι δυνατων between the Megaric Diodorus and Chrysippus the Stoic which Cicero refers to in his book _De Fato_. Diodorus says: “Only what becomes actual was possible, and all that is actual is also necessary.” Chrysippus on the other hand says: “Much that is possible never becomes actual; for only the necessary becomes actual.” We may explain this thus: Actuality is the conclusion of a syllogism to which possibility gives the premises. But for this is required not only the major but also the minor; only the two give complete possibility. The major gives a merely theoretical, general possibility _in abstracto_, but this of itself does not make anything possible, _i.e._, capable of becoming actual. For this the minor also is needed, which gives the possibility for the particular case, because it brings it under the rule, and thereby it becomes at once actual. For example: _Maj._ All houses (consequently also my house) can be destroyed by fire. _Min._ My house is on fire. _Concl._ My house is being destroyed by fire. For every general proposition, thus every major, always determines things with reference to actuality only under a presupposition, therefore hypothetically; for example, the capability of being burnt down has as a presupposition the catching fire. This presupposition is produced in the minor. The major always loads the cannon, but only if the minor brings the match does the shot, _i.e._, the conclusion, follow. This holds good throughout of the relation of possibility to actuality. Since now the conclusion, which is the assertion of actuality, always follows _necessarily_, it is evident from this that all that is actual is also necessary, which can also be seen from the fact that necessity only means being the consequent of a given reason: this is in the case of the actual a cause: thus everything actual is necessary. Accordingly, we see here the conceptions of the possible, the actual, and the necessary unite, and not merely the last presuppose the first, but also the converse. What keeps them apart is the limitation of our intellect through the form of time; for time is the mediator between possibility and actuality. The necessity of the particular event may be fully seen from the knowledge of all its causes; but the concurrence of the whole of these different and independent causes seems to us _contingent_; indeed their independence of each other is just the conception of contingency. Since, however, each of them was the necessary effect of _its_ causes, the chain of which has no beginning, it is evident that contingency is merely a subjective phenomenon, arising from the limitation of the horizon of our understanding, and just as subjective as the optical horizon at which the heavens touch the earth.