Occasionalists, he goes on to metaphysics in its sense of ontology, as Mill never did. The difference here is that he does not clothe his metaphysics in a theological dress. In the later development of his thought, Schopenhauer dealt more expressly with the question, how this kind of phenomenism is reconcilable with a scientific cosmogony. On one side the proposition, 'No object without subject,' makes materialism for ever impossible; for the materialist tries to explain from relations among presentations what is the condition of all presentation. On the other side, we are all compelled to agree with the materialists that knowledge of the object comes late in a long series of material events. Inorganic things existed in time before life; vegetative life before animal life; and only with animal life does knowledge emerge. Reasoned knowledge of the whole series comes only at the end of it in the human mind. This apparent contradiction he solves by leaving a place for metaphysics. Our representation of the world as it existed before the appearance of life was indeed non-existent at the time to which we assign it; but the real being of the world had a manifestation not imaginable by us. For this, we substitute a picture of a world such as we should have been aware of had our 'subject,' with its _a priori_ forms of time, space, and causality, been then present. What the reality is, is the problem of the thing-in-itself (to use the Kantian term). This problem remains over; but we know that the metaphysical reality cannot be matter; for matter, with all its qualities, is phenomenal. It exists only 'for understanding, through understanding, in understanding.' These discriminations made, Schopenhauer offers us a scientific cosmogony beginning with the nebular hypothesis and ending with an outline of organic evolution. This last differs from the Darwinian theory in supposing a production of species by definite steps instead of by accumulation of small individual variations. At a certain time, a form that has all the characters of a new species appears among the progeny of an existing species. Man is the last and highest form to be evolved. From Schopenhauer's metaphysics, as we shall see, it follows that no higher form of life will ever appear.