In these works he attacked the existing theories of externality which to the unphilosophical mind is proved by visual evidence.
As cause of our sensations and ground of our belief in externality, he substituted for an unintelligible material substance an equally unintelligible operation of divine power.
It was evident that a similar analysis might have been applied to tactual consciousness which does not give externality in its deepest significance any more than the visual; but with deliberate purpose Berkeley at first drew out only one side of his argument.
This externality of religious truth to the mind is fundamental in scholasticism, while the opposite view is equally fundamental in mysticism.
After explaining that he will use the term "external world" in the sense of absolute, self-existent, independent matter, he attempts in the first part to prove that the visible world is not external, by showing - first, that the seeming externality of a visible object is no proof of real externality, and second, that a visible object, as such, is not external.
In the Principles of Human Knowledge, externality in its ultimate sense as independence of all mind is considered.
The battle of his life was on behalf of personal religious experience, in opposition to the externality of rationalism, orthodoxy or sacramentarianism.