In his Monologium, St. Anselm had proved that God is, and determined his attributes by way of induction from the ideas in the human mind, but it would seem not wholly to his satisfaction, or, at least, that in writing that work he discovered, or thought he discovered, a briefer and more conclusive, method of demonstrating that God is. He had already proved by psychological analysis, in the way Cousin and others have since done, that the human mind thinks most perfect being, a greater than which cannot be thought. This he had done in his Monologium. In his Proslogium he starts with this idea, that of _ens perfectissimum_, which is, in fact, the idea of God. "The fool says in his heart there is no God;" not because he has no idea of God, not because he does not think most perfect being, a greater than which cannot be thought, but because he does not understand that, if he thinks it, such being really is. It is greater and more perfect to be _in re_ than it is to be only _in intellectu_, and therefore the most perfect being existing only in the mind is not a greater than which cannot be thought, for I can think most perfect existing _in re_. Moreover, if most perfect being does not exist _in re_, my thought is greater and more perfect than reality, and consequently I can rise above God, and judge him, _quod valde est absurdum_. Leibnitz somewhere remarks that this argument is conclusive, if we first prove that most perfect being is possible; but Leibnitz should have remembered that the argument _ab esse ad posse_ is always valid, and that God is both his own possibility and reality. Cousin accepts the argument, and says St. Anselm robbed Descartes of the glory of having produced it. But it is evident to every philosophical student that the validity of the argument, if valid it is, depends on the fact that ideas are objective and real, that is, depends on the identity of the ideal and the real.