Conditions of truth:

1. A thing to be possible must involve no contradiction.

2. A thing of many things to become realised, that is to say, the (actual), real of many possibles, must have a sufficient reason for its realisation.

We owe these two laws to Leibnitz.

The Law of Possibility is internal to a thing; that is to say a thing contains in itself the possibility of its own existence. But nothing contains in itself its own existence; the reality of a thing does not depend upon itself. Thus the Law of Reality is external to a thing.

Hence are taken two other laws which concern the universe more, and are more near to us. These are called the Law of Life and the Law of Persistence in living.

Thus an animal, once born, has a possibility of continuing to live unless such a conception involve a contradiction.

Again, and in view of the struggle for life, the second law, the Law of Persistence in life causes the animal to depend no more only on itself, but on the environment and in its adaptation to the environment, etc., is the sufficient reason of its existence.

These 2 laws may be reduced to one, under 2 names: the law of internal contradiction and the law of external contradiction.

An animal that is born has possibility of persisting in life (Law of internal contradiction, for, since it is not dead it can go on living). But if it be born too weak for living, it cannot struggle against the environment and must die (Law of external contradiction).