Argument for God
In PHK and 3D Berkeley offers two original proofs for the existence of God. The first, we can call The Argument from Causation, the second we can call the Argument from Perception. Only the first one appears in PHK, while both of these appear in Dialogue Two.
The Argument from Causation
This argument proceeds with the observation that my ideas of sense (as opposed to be my ideas of imagination) are not caused by me. Since they must be caused by some mind or other, they must be caused by some other mind, and this other mind is God.
In drawing this distinction between the vivid and forceful ideas of sense (caused by God) and the faint and weak ideas of imagination (caused by me), Berkeley also distinguishes reality and mere thought (appearance). Ideas of sense constitute the real, natural world while ideas of imagination are the thoughts produced by me.
(1) Sensible ideas must be caused by some spirit
(2) I am not the cause of my sensible ideas
(3) So: There must be some other spirit which causes my sensible ideas
(4) This spirit is God.
The Argument from Perception
(1) Sensible things cannot exist unperceived
(2) Sensible things exist independently of my perception
(3) So: There must be some other spirits which perceives my sensible ideas
(4) This spirit is God.
This argument proceeds with the Berkeleian claim that sensible things cannot ever exist unperceived (i.e. his basic idealism). The second claim is that sensible things exist independently of my own mind (i.e. exist independently of being perceived by me). From this it follows that there is another mind which perceives sensible things (e.g. when I don’t perceive them) keeping them in existence. This other mind is God.
The first claim in this argument depends upon Philonous’ arguments in the First Dialogue that sensible things are mind-dependent. Recall that this forced Hylas to become a skeptic (i.e. to deny that reality of sensible things). By contrast, Philonous uses this claim of mind-dependence to help advance this original proof of God’s existence. Notice that he spends considerable time saying how beautiful everything is and that men who deny the existence of such sensible things deserved to be laughed at.
It is an interesting question how Berkeley thinks he can defend the second claim (i.e. sensible things exist independently of my own mind). One natural thought is that this is simply an appeal to common sense (“Well of course that tree exists even when I don’t perceive it!!!! That’s only common sense!!!”). One concern with this is that Berkeley’s very own position appears to undermine this common sense view of the world. His subsequent appeal to it would consequently appear ad hoc. That said, more can be said in defense of this move.
If Berkeley is right that sensible things are mind-dependent, then we can agree that one of two things occurs when we leave a room. (1) Everything in the room ceases to exist; or (2) Some mind (God) exists and perceives those objects. So which position is closer to common sense? Surely the last position is more common-sensical (especially if we recognize how many human beings actually do believe in God already). Since the whole game between Hylas and Philonous involves staying closer to common-sense, this commitment can further motivate option (2).
However, there is a deeper reason in favor of the view that the sensible things I perceive are independent of me is that, since they are caused by another mind, then they must exist in that other mind, and can consequently exist in that other mind, independently of me. This suggestion would help connect The Causation Argument and The Perception Argument together.