It was by the help of this learned science that Kant broke the serried ranks of his antagonists. Confronted by two philosophical opinions, both of which he considered erroneous, he proved to the materialists Condillac, Hume, and Locke, that there is something within us which could never have been supplied from without, which therefore belongs to our ego, that is to say to the subject thinking and not to the object thought of, or matter; then turning against the Idealists of the time of Berkeley, he shows that there is something without us which could never have been supplied from within; and when he proved that intellect and matter are correlative, that they exist for each other, depend on each other, form together a whole that should never have been torn asunder, two streams of philosophic thought, which had been running in separate beds, met for the first time.
The existence of the phenomenal world being proved by the irrefragable testimony of the senses, is admitted also by reason, and, as a necessary consequence, another, not only in appearance, but which _will be_, assuredly; as sound is independent of our hearing, as material objects are independent of our sight; for though Kant declares our inability to know objects as they are in themselves, he does not deny their existence, since he says, “We should be capable, if not of knowing things as they are in themselves, at least of knowing them as they are to us, otherwise we should arrive at the irrational conclusion, that there may be appearances without something that appears.”
Kant undertook to make an exact science of the necessary and universal ideas of the human mind, such as logic and mathematics, which are parts of human knowledge; to this end he wrote _Critique of Pure Reason_, afterwards he composed another work, the _Critique of Practical Reason_. Practical reason may also be called pure, in as much as it does not allow itself to be influenced by anything but what proceeds from itself, and reason becomes practical when it seeks an independent principle which determines the will. This principle is formulated by Kant in the following terms: “Let each individual follow commands which may be considered as a universal law imposed alike on all human beings.”